TasNat 1925 Vol1 No4 Pp3-10 Lewis OutlinesGeology

7/28/2019 TasNat 1925 Vol1 No4 Pp3-10 Lewis OutlinesGeology

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THE TASMANIAN' NATURALISTtions of rain bringing winds have littleeffect.

However they are caused deserts allpresent a peculair type of erosion. ~ h . eeff.ects of water are reduced to a mID i -mum and the effects of wind are increased and allowed full play. The actIve agents in wind erosion are the particles of sand blown against the rocks.In dry regio.n.s the rocks are worn awayby this rubbing. In time the softerportions stand out. Much of the northof South Australia consists of strata ofquartzite with some layers much harderthan the rest. These beds are tiltedat an angle of about 45 degrees, and thehard layers recur every five miles orso; also. they run parallel to the l i n ~ ofthe prevailing' wind. The soft portionshnve been worn ont by the wind carriedsand and these hard layers now standout as walls 20 to 100 feet high, andrunning for miles across country. Thisis Quite a common feature o.f desert topography.

As the wind has more cutting powerjust above the ground level, where .itssand content is highest, isolated hIllstend to become cut away at the baBe,and thiR very sharp featured, or evenRomE'what m.Hthrc.om shaped. F o ~ m sare given which water could nevet' glve.These precipitolls s i d ~ s and. shape facesare always on the slde facmg the prevailing wind, the leeward being uneroded and gently sloping. A landscapedotted with hills all shaped in this way,and with the gently sloping sides all poil.'.ting in the same direction, is a certaill mdication of past desert erosiM. Notani\' do the 80lid rocks become worn bythe" sand. bllt also loose boulders andpebbles are a!feeted. They become f a c e ~ed into irregular faces and polished, moreso on one side than another. These peculiar features of the desert are termed"Gibbers" in Australia from the nativename for a stone, and are very typical.Great accumulationR of wind blown rockfragments, termed "laess." ire another mdieation or dry localities.

Lakes and Marshes_A lake indicates an "a.ccidAnt" in thedevelopment of the la.ndscape. A mooment's reflection will satisfy anyone thatwatAr erosion, working as it does gradu.ally back from the river's mouth coul,lnever hollow out a lake basin. To do

JlO would require the river at some stageto run uphill. Further, lake. indicatethat the accident to the orderly cour8eof erosion has occurred in t.he not fardistant past. The exilltence of lakes is

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ephemt'ral. Rivers may soon either cutaway the dam empouudm.ll the lake orfill jt with silt.'fhe c o m m o n e ~ t cause of tht: formatI.onof lake basins is ice action. A glae.lerscoops hollowB in t ~ c r o c ~ s o ~ E : ' t ' \ ~ . h 1 C hit passes in a way I m p o S S l h ~ e for \ ~ a t ~ : .Also a glacier drops all Its mm ~ l I a lload, where i t melts. This very o ~ t c nforms a dam of rocks and c.lay rIghtacross its valley. When the Ice m e l t ~water accumulates in the hol1ws, t h n ~formed as lakes, Most of the I m I ? o r t a l ~ tlakes in the world \vere formed In t l l 1 ~way. All in Tasmania, with the exception of Lake Tibel'ias, Lake Dulverton,Gl'imes's Lagoon. and perhu-ps L J ~ ( 'Crescent and Lake Sorell are due to I C l ~action so are the great lakes of North .ern America, Russia and Siheria. ~ h enext important method of lake formatlO'nis by a relatively rapid a l t e r ~ t i o n of leveldue to earth movements. EIther a H o ~ kof land sinks into what is called a nftvalley, and as a result portion is left" basin with no outlet. The salt andmud plains marked on the maps of f'us-tralia is Lake Torrens, Eyre, Gardmer,and others in South Australia; a l ~ o thegreat lakes ()f Central Afriea. and theBlack and Caspian Seas, are so formedOr portion of the l a n d s c ~ p e m,ay be ':IP-lifted, and at the same tIme shghtly tlltpd. this again interfering with drainalle,Lake Tiberlas and Lake Dulverton. andthe swalnpe at Mona Vale and Cress:\'are due to this cause,I,akes may also be formed by ' otheraccidents such as the blocking of a valley by lava flow, and small ones evenby a landslip. Again, a river may buildi t . bed up so high that tributaries calloot reach it as is done in N.W. Victoriaand along the Darling, or the sea windsmay block a river's mouth with sanddumes, as at the mouth of the Murrny, and many river mouths in Tasmama.although strictly Rpeaking there are morearms of the sea than lakes.Mnrshes are merely regions formed ina similar way to Jakes, but either not soperfectly formed or nearly obliterated byriver erosion or derosition of sediments.One common illusion must be dispelledhere. I t is often said that vegetationmay fill in lakes, and Lake Tiherias, withits weed-covered surface is instanced asnu example. This is a fallacy. Vegetation cannot grow in water over about6 feet deep, with the exception of smallaquatic plants which would not help materially to fill a lake basin. Plants donot cover a lake's surface until the water


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THE TASMANIAN NATURALISTlevel has been reduced by other c i r ' c u m ~stances almost to vanishing point. Lake'l'iberias is a reed-covered marsh and nota lake. Marshes, butter-grass plains, peatbog's, etc., are typical of glaciated COUI1-try. and owe their origin to similarcauses in glacial lakes.

'rhi. brings to a close our all readyuver-Iong chapter In. The maiu agencies at work developing our landscape havebeen pointed out. All possible sxamplescannot be given, but every. form exceptperhaps the desert, can be observed I I ITasmania. I t is only necessary to lookabout you during any bush walk to ' seepumbers of the phenomena describedabove. The student should, after mastering the principles 0. landscape development, pau&e during his walk and sayto himself, t.o what agencies were theselcatufes 1 see before me due? This study01 physiography IS the basis of praclI'cal geology, and the most important partor uur subject. The student should ma.ster it before proceeding further.

The writer would have pl'eferred to placeexamples of the above described featuresill the text and in a hopedfor re-issueof these articles this will be done, but atthe present tiJne has i l0t allowed thecollection of aH the available examplesand a r-ea1SOuable choice cannot be made.Also these examples arc so numerousthat without a judicious selection theywould over.load and confuse the text.There is the additional reason that atpresent there is a contl'O"ersy as to whether our landscape is due to differentialtiiaL'J.se intrusion followed by erosion ofthe softer l'ocks, 01' wh,ether, as the writer thinks, it is due to subsequent blockfaulting. Until this point is definitelyagreed upon many examples will be COIltl"Over::!ial. All that can be done here isto give a few examples of the features de!o:Icribed above in illustration. No attempt is made to give a full list.The effed of l'ise and :all of the land01 ' the level of the sea may be observedalong all the lower levels of our l'iverl:i.Our harbol's are mostly drowned valleysworn out by water and then flooded bythe sea. The flooding has caused the deposition of sediments high up in the estuaries wher- the current IS now checked.The Den\oent. Huon, Tamar, and Jler'l5cyare fine examples of drowned valleys.\Vhell the sea WaB much lower during ther,ecellt lee age the l ' i v e l ' ~ cut these valley-t!. :N{)W they l'annut carry th-eil'I:ledimellts beyond Bridgc-water, Frallklin,

Launceston, and Latrob2, ':""'hel'e extensivemud flats and flood plains are now beingsuperimpoesd on the old valley floor.In many places a subsequent partial re juvenation may be seen to be commencingand these and most other rivers can be8eell cutting through their flood plainsof river gravels. This is especially noLceable on the Derwent between .Macquarie Plains and New N orfo]k, where oldtiood plains consisting of the typical riverconglomerates are standing many feetabove the high

c ~ ) U r s e of erosion above it , and given POl' tlons of the valley appearances of maturity. \Vhen the river has cut throughthem it has cut through the Hood plain.sdeposited higher up, 'Vhen the SouthEsk cuts through the hard diabase ba,'at the Cataract Gorge it will deepen it supper reaches very quickly in the Bofterrock further south.. The bulk of 'Va.smania's drainage isI I I the youthful stage. In fact, Tasmaniapos-sesses an area of juvenile drainage inexc-ess of tha t found in mOBt parts of theworld. With the e x ~ e p t i f ) n of the largestI'Ivers, the drainage is in tha earlieststages. This points to the conclusiontha.t. water has not long been at workerodmg our present lands-cape. :Most ofonr creeks are mere mountain torrents,and largely depend on :lCtual ra.infall fo rtheIr continllanc>e. Waterfalls and lakesare very common, and the exist-ence ofmany inJand cliffs, apart from water

c O ~ l r s e s a n ~ glaciated CCllutY'y supportstIllS cOnCh1.Slon. A Boft rock-like sandstone WOUld. not stand 33 a cliff duringmany geologICal ageoS, and these cliffs ilrecommon aB over the country, Good examples eXIst behind Lindi.:sfarne and Hisdon, on Mt. Faulkener. and south ofNew N(}rfolk.The rivers such as the Derwent HuonGordon, King, Pieman, Al'thul' n{wkg]!s, BIyth, Forth, ~ f e r s e y . Sout'h Esk,' andRmgaroocn.a have reached a somewhatwore mature stage. But even these aremere]y a series of deep reaches separatedbJ; rapids. None Rife strictly navigable.Rlve-r,s with as much water as these inEngland, France or Germany would beg ~ e a t internal trade routes. But still theserIvers are in a more advanced stage thanmere youth. The writer suspects thatriver erosion has not been entirely responsible for the development of thetopography of these valleys. They are proba,bly due to large block fault., and vhet ' h ~ e r s have found courses in them s o m e ~


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'tHE TAsMANIAN NATURAi.1S1'what ready made at a fairly m.ture stage.The streams have been at work for a considera:ble time, and have further mouldedthe contours. Thus these wide valleyshave the characteristics of landscapes a-pproaching maturity. but as SOon as youmove ou.t of them the drainage is in thefirst stages of youth. This gives us adouble set of l


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'.tBE TASMANIAN NATURALISTwe must find out what kinds of rocksoccur, and - how they differ from eacho.ther. We must then discover why theyoccur and account for these differences.This is the intensive part of our subject.\Ve turn from the broad form of theJandscape to the minute structure of eachportion of it , from, as it were, the telel:ieope to the microscope, but always rememberillg that as geologists we are ll?tiutcl'ested ill structure of rocks for Itt:!eIf hut as an assistance to, as a proofof ~ u r theories of the origin of ourlandscape.

Differences in Rocks.III cxuuunmg two lneces or rock they.. JI i \'ery I.Hteu be lOUllU to oe dlttel 'ent.LI:'IUdU): tl1J8 will be suthelently marked(.u ue UH::iLenllble by the eye a t onee..llll.'t:!e Ulnerences are ultfCl'enCes ofc;elleJ.'dl ul>pt:!aI'UIlCC--su'ucture, haruu,e::l.s,LOiUJ.', leel tu the touch or lUestl'e. l ' 1 H ~

U t l l l S ~ ut (iIllCl'Cllce.s may be due to dulCl'eut,;C.s H l compml1tioll 01 ' in Illode o.ilUrUHlLlOll . having determined thel;


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'tHE TASMANIAN NATURAL1STwhich have been altered from those theyolllce possessed. The original rock mayhave been igneous or sedimentary, andthe change may vary 11'0111 a tllight h a l ' d e n ~lUg to a complete cJlange of chenucal compooition, and structure to a ll extent whichobliterates all idea of the original type."Metamorphic" indicates heat as the chiefagent of this change. The term "schistase" is unsuitable as many metam-orphi.trocks do not _possess this characteristic.But a's has peen said, rocks are termedeither igneouB, sedimentary, or metaplOrphic, according to certain' defined r u l e ~and quite independently from the meaning of the words which are now technicalterms. Mineralogy.

Before we can proceed to cla..saify r..>cktypes we must study their characteristicsminutely. The portion of our sciencewhich do.. . this is called mineralogy, I tclassifies the minerals which tO$ethermake. up the rock. Mineralogy has developed into a science of its own, andrequires mUlch equipment, such as micro

s ~ o p e s , and ordinary laboratory service:;.I t is quite beyond the scope of the!olcarticle6 to delve into thi..s wide and fascinating study. All that can be atte:npted here is to give a classification of themore important rock-forming minerals.This must be done as much of our laterclassification of rock types depends .:mthe presence or abseThl'e of certain common minerals. \-Ve must eaU on the specialised knowledge of the mineralogist togi\'e us what assistance he ean to determine the origin of our rockt-!, informatiollgained by the intensive study of the chat"actel'istics of the vnrioUls component partsof our rock.

SECTION 19MINERALOGY

Rocks are made IIp of collectlolH!I ufmineral particles in all indefinite mixt Ul'eof crystals or grains varying greatly i llSize, shape, and chemical compmdtion.Some few rocks are made up of grainsall of the same mineral, a greater numbel' consist of grains of twn distinctmineral species, but the majority of l'OCkHure a mixture of three or more minerals. The rocktype depends OI l tht'type of these constituting minerals, andthe mineral type gives an indication a t ;to the conditions under ,vhich the rockwas formed.HA mineral is a body produced by theprOCE'sse! of inorganic nature, having a(8)

u e f i n i t ~ chemical composition., and If1Ol'med under iavoraole conditions, acer'LillIl ehara


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THE: TASMANIAN NATtJRAUS1'Therefore a crystal is not somethingrare and striking, but the normal condi-tion of inorganic matter which has scliditied from a gat> or liquid, and all r o c k ~so formed are an interlocking network of

crystals, and each crystal is a definitemineral. Other rocks consist of mineralsbroken from their parent formation, and1 e-cemented together later. Again, othercrystals of the same or different m i . n ~ r a lmay be insinuated amongst the ongmalcrystals subsequent to their formation.

Formation .. Crystals.All liquids in nature consist of a mix'ture of m()re than one element (mercury.a very rare element in nature. being

an exception). Certain elements haveaffinities for others with which theycombine in accordance with definitechemical lawe and form chemical com-pounds, which on crystallisation. gi\eus mineral species. Differen t nuneralsmelt a.t dlfferent ternperatt.res (c.f., ice,lead iron, and asbestos), and i t ff that solid will be deposited.An alteration in the composition of theliquid, or iiB temperature, wHl alter thesaturation point (some things can bedissolved by hot water tha t cold waterwili not atl'ect).Now imagine a liquid made up ofmelted rock in which are certain elp-rnents which commonly compound intothe minerals. A, B, and C, and whichmixture is gradually cooling. As thetemperature ckops Cl point will be reached at which either one of the mineralsfreezes or the liquid becomes satt:..ratedwith one of them and deposits particlesof it. Probably both processes will beproceeding as to different minerals atthe' same time. In both cases small ~ m 1 i dp:trticles of this mineral win be formedin the mixture. I f it remairu; liquidlong enough these particles wili growit"! more of the minocal .substance is

; ~ d ( k d by the effect of the cooling, andwill form crystals. The withdrawal ofthl'.se substances will alter the composition of the liquid. This may caUiSe the]IH'l'i111tation or crystallation of anothermilleral who:;,e crystals will grow alongside the t ir8t. T1' the mass remainsiiquirl long enot,/:.;h every particle will( t ' y s t a l l j ~ e ollt 11I:n proper miJwrllJ,and the whole will be an illter:lo('kcdmass of C'l":t-Itals of (lifferent minerals. I tfollows that the {'ompound,s predomInating in the liquid :lDd those whieh

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solidify at the highest temperatures willcrystallise out first and the other min-erals will grow round the first fOll:nedcrystals and in the interstices betweenthem. also the lower levels will be rich('st in the mineral that first ('rystalli8eJ:!owing to the sinking of the crystals .and to precipitation of the minet::u.1. If themixture is 80 compounded that in spiteof different times of commencement each'mineral fimshes crystallising at the sametime, which must be at the same in-stant as the disappearance of the lastdrop of liquid, the' minerals are


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THE TASMANIAN NATURALISTawl give a great variety of figures. Forthe shapes of these figures the readermust consult a text book of mineralogy,as a discussion of them "is impossibleexcept at great length and. with thea.id of a multitude of figures and dia?;rams. The determination of a mineraluy its cl'Ydtallogl'uphic: forlll is often (hf- ..tlcult and very rarely t.:sed in practice,the forrm having been worked out fo rall known minerals, and is more easilyascertained by reference to a text book

than by actually working it out frOInthe princ:iples of cl'ystallography.Beside3 these possible forIns, othersarc introduced by the habit pos:-ressedby crystals of certain minerals of twinning, t Int is , of growip.g together illpair;:; 01 ' more numl::!1'OUS aggregates.Many minet:dlli adopt very characteristict\o,.'in fOl'llls by whieh they can be distinguished. A. N. Lewis

Life of the SwallowOf all the slimmer migrants who regulal'1y visit our shores, p e r h a l J ~ nOlle afford th e lover of bird life 60 muchpleasure as the swallow.The arrival of the first swallow stirsthe pulse vf every lover of ~ a t u r e , recalling as i t does, perhaps, certain pleas_ant memories, and impresses llPOll us thefact that summer-or a t any l 'ate tnemontht3 termed by courtesy summer-ISalmost upon us, while the departure of

the beautiful bird warns us of the depressing fact that another winter ifldrawing on. It is , of course, generallyknown that some individuals allow theirmigratory impulses to lapse and re,mainwith us right through the year, as de..some of the cuckoos and other birds affect.ed uy migration; but the great bulkof the swaUows go away, usually to NewSouth ""ales and Queensland. at the approach of winter.It is not the beauty of the swallow

ulone that attracts our attention, but! ~ ~ i ~ ~ ~ ~ ! s ' ~ i ~ d ~ : ~ i t ~ e n e t n ~ ! : r b t t ~ a d : ~ ~ ~ ~for he often shares wi th us the housewe liVe in, and claims the Drivilegesof a welcome guest during his too shortI-ltay; and he is well entitled to ourlove and protection, for, being a purelyiut':lf'ct feeder, the amount of e;oCJcl hedoes is incalculable.

Designed for Flight.None of our bird v i ~ i t o r ' S are moreadmirably and beautifully designed for

~ o : ~ ~ ~ d t ~ ~ n ~ ~ ~ f l ~ ~ l \ ~ ~ ~ e t ; h ~ ~ e t l ~ x : e ~of wing go's to show what an impCJrtant lJart this member plays in the lifeof the bird. On the other hand, theircomparatively small legs go to provewhat a t;maIl proportion of work theyare t:aIled upon to perform. For pra

TasNat 1925 Vol1 No4 Pp3-10 Lewis OutlinesGeology

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Transcript of TasNat 1925 Vol1 No4 Pp3-10 Lewis OutlinesGeology