MINING

Otago Witness, Issue 956, 16 May 1889, Page 11

MINING

POPULIR BIItflH6(

By J. A. Miileb.

X. ' (Cnoehmon.)

In the determination of the epochs of geological time the fossil, remains found embedded ia the rooks serve as.'yearß do in the record of human history.!, The term "fossil," for, this purpose includes everything that may be " dug up," which is the -literal meaning of the word " fossil," and also all objects which in any way shed a light upon the past history of the rocks. " The trails or burrows of a worm, preserved ia sandstone and shale, claim recognition as fossils, and indeed are commonly' the only indications to be met With of the existence of annelide' ' (ringworms), life among' old geological formations. The droppings (coprolites)" of fishes and reptiles are excellent fossils, and tell their tale as to the presence and food , of vertebrate life in ancient waters. The lifctle aggleatihated cases of the cadisworm remain* as fossils' in 'the formations from which perchance 'most', other 1 .traces of. life may have' passed away. Nayi the very handiwork of man, when' preserved' in , any, natural manner, is entitled to rank' among fossils, as, for instance, where his flint instruments have been dropped into the prehistoric gravels of ' river valleys, or where his canoes have been buried in the silt of lake bottoms."

Sometimes even the animal or plant entombed has completely passed away, leaving nothing but its mould, which, if dissevered before it is totally destroyed, may be filled with plaster of patis, and an exact image of the original animal or ' plant obtained. How these organisms got, into the rocks is explained in many' ways. iPeat, mosses, cares, river beds, and especially the delta at the mouth of .rivers, lakes, mineral springs upon land, or in the, ancient seas and oceans, dead and dying animals .and plantsdrifted and lodged into positions where they were buried in the depositing silt- Once entombed the preservation of .these remains depends chiefly upon their original 'structure and and, also upon the manner of! their' entombment, and 'subsequent ■chkng^,. , Animals provided*'" wijih' hardvbpny .sum&6)rlafg'e ! 4n4 weTi-devaopjßfif framed, were more 1 likely to leave' (ie^iti^jihem' some of weir '•fieA^'^^tMaft^MSte^wmSqijßa of 1 gela^n'ow 'ga^nc^^prbvja^^the^^iiucsJ^

changes' acting upon'Hh'e liard.et^Srts'weWindt 0$ 'such a nature as* ( t6 'destroy the'ni. " - The 1 manner of preservation is of three kind's. "Firsf, the original substance, is partly or..wKully preserVedi The : frozen mammals of Siberia, and . insects in the resin of plantsi'arefaniiliar illuS" 1 trations of this' kind." Secoadly, the' original* gubstance is entirely renibved, leaving, merely the external form, particularly common in sandstone rock. And, thirdly, the, original substance is moleotllarly replaced' by mineral wat'erfwith •partial or entire' preservfttiori of the internal 'struotare of the organism; This is the only' true '.' petrifaction." !> The process consists' in the abstraction of the organic, substances, 1 molecule by 1 molecule, and in their replacement by precipitated mineral matter. So gradual and thorough ha3.thiß interchange often been that' the minutest structures oflplant' and animal have 'been preserved perfectly. Silicified wood is a familiar example. A large number of minerals serve this purpose of replacing original substances, and also metals,', such as native copper and native silver, sometimes have taken - the place of the original OJrganifim. By the aid of; fossil* remains; it is possible. to determine whether the 1 system of xocks in, which they were found, belong to a marine' formation, or a freshwater lake, deposit; whether at the time of its interment it was dry landj or a deep .sea basin, and so on. ... < There is a much greater uniformity between the fossils of a geological epoch than there is between the rocks of < the same age all over the world. However, as a certain relation between , the organic life of any period or system and the nature of the prevailing rooks of; the same period' or system is traceable, it seems that plants ' and animals "in their features have changed much more. with the advancing age of the world than with the construction or composition .of the -rocks. This is chiefly .determined by certain fossils, known as " type fossils," which, although not always the most numerous, have some strongly marked characteristics by which they are easily, distinguished from ,, all other, fossils, and these type 1 fossils 1 are always taken, to define the age, and according to the age the place the' rocks they belong, to occupy in. the order of their superposition. 1 Obser^a?, tions'.made over, large portions and in .every quarter of the globe have enabled geologists to ; divide the stratified parts of the earth's crust into divisions, systems, formations* and groups, which by their name -express their age as well as the order of their' superposition. But in; neither age nor .superposition is there any.possibility of 'defining' within exact limits of either time or space any of the demarcations, whether they be taken as divisions, systems, formations, or groups, - All this shows the important part fossils play in general science, and as they have also a very significant application in practical mining geology, a few remarks may be made on the .search for fossils. They occur most frequently in limestones, calcareous shales, and in clay beds ; they are scarcer in sandstone and conglomerate, and all the older metamorphosed rocks,- antil j they appear to cease altogether in the rocks of the Archaean division. "Some districts' are much more productive ' in fossils than others, even where the rocks correspond in age and composition. The greatest' number of entire or complete fossils have been found in the faces of exposed' precipices, where the rock has been worn away by wind and rain, disclosing the fossil to the view.' Mining exploitations haye added valuable fossil remains, but owing to the nature of the work, and the carelessness of the workman,-much that is of the greatest importance to science in thist'tesp^ct is either destroyed. ,or passed by unnoticed; ,A greater amount. of ( care ( in the treatment of rock than obtains at present can- hardly be expend from pc miner, but !be may be_ recommended with advantage to himselif to devote, a little more .attention ,t§ the, • stuff broken, out jq t Ws. operations', ,j,nd if a * & U. possible to olos^yjins^^^useams'^ah^. pollec^ 1 lions in . which, f osail remains t if epresented, and, .then, trust to chance for any discovery he may Eiftke, ,' „ i, •,' '* „ J The wienjafic process of -looking for fossils Is, besides inspection, of exposed faces of likely (rocks,( rocks, tq 0911,696, sift, and mash portions of the 'debris crumbled and ins'pect j the rubble witih a microscope. ' In .this manner, many fossils in a very reduced or mutilated etate have been found, and proved of, the greatest assistance and value in geological researches. „ ; |i - < Next' to fossils the minerals contained in the rocks' are of ; the greatest value in determining their ages aridpompositipns. Chemical research has shown not only that certain minerals are composed of certain elements in fixed and un^ 1 alterable proportions, but also that they all'undergo changes, a kind of life-like imitation of growth and decay. In,jfchese chahgea- strange combinations 1 have resulted. Small prystalsbf silicate of sulphur* attach themselves to large crystals of calespar. In some cases parts of the same mineral arrange themselves in such a manner that in a circular druse of quartz all the crystals turned, outwards' ( are smooth, whereas all corresponding planes of , the same crystal pointing inwards, or, towards, the centre of the druse, are rough, and covered with numerous small crystals. Again, in, other instances, one or more species of minerals are r so ingrown that the axes of the different crystals 'disclose a certain correspondence," showing, that the different crjatals exert an appreciable amount of influence one upon the other, as if one crystal had dislodged another and entirely or partially usurped its place, or that one mineral compelled another to lean agajnst itself accordingonly to its own planes or in the direction of its own axis. '"' When the minerals of one kind of rocks are> carried by the circulating waters into neighbouring rock's of a different, composition, the changes yrorked in the invaded rocks ,are not only much more '.complicated,' they are also much more extensive. Whole mountain ranges' have been charged' in this manner with minerals entirely ne,w io them. By this invasion the native ! minerals .have been compelled to form new alliances, or have been expelled .totally, their places being occupied by new* substances "so completely changing the ; rocks over large dis- J tricts that nothing' remained by [which to 're- j cognise' their original ( nature but i'pferhaps' the j fossils the original, rocks contained. There are strong reasons to believe that some of the known oldest rocks, as those which prevail on most of the Otagan goldfields,have been metamorphosed in this manner., Peat bogs "changing into beds of bituminous' coal, limestone # deposits into silica or ironstone, are familiar illustrations of the same process. Sometimes such metamor- j phoses have been accompanied by a shrinkage in the bulk of the rocks, or under different circumstances a swelling or considerable increase has taken place- in. the bulk of the metamorphosed masses, resulting in depressions or elevations extending over large portions of the earth, showing that one and the same cause may lead to^idely differing results.' , , pb soon as a sufficient number pf observations have definitely determined the order of euccession.of the 'different metamprphoses of. minerals, it be possible! tjp\ say with 'cer^a^, how each' djffe^reot ' change succeeded . its \f^, /decessqir;' 1 ¥ill it be possible j;q ; '^ay s< iiß^r,' tocji, or this laye,r of jock grew and ipcreased ai i cer^ift ti^^Wi^ea^ t^j^j%»| itself into its aWms., When, the minerals armed ' iM' tiW'ktiM&Wiwti he'islble tg'deteri '

mine by simple'and inexpensive tests the'valne of an'ylodejandis in' the' position bylreUable 'scientific', data 'to' distinguish between its barren and' profitable) ' portions, when he J has 'a'ccftiired the knowledge of acpurately &ceraining|'tlle law' of ' breaks in 'lodes,' and can' measure ,by r correct calculation the amount of the"; dißpla'cement, then will mining rank as a -respected profession and a recognised commercial industry! Nor. is this consummation far distant. With every year progress towards I 'the goal is made. New discoveries shed floods of light upon the secret workings' of Nature; out of the depths of the I ' rocks sounds the, voice of aeons' of time; pasf creations now turned into' stone i tell 'of the ever-active forces that built up and 'maintain the world. . '