POPULAR MINING GEOLOGY.

Otago Witness, Issue 1951, 11 April 1889, Page 12

POPULAR MINING GEOLOGY.

VII.-'THE METAIVrORPHOSES OF THE ROCKS. In the foregoifig chapters it has been shown that the world is maintained by change. No absolute test is known in any one division of the •-economy of Nature. Not only does this constant •change go on upon the surface fend in animated mature, but withiu the ernst <of the earth which *we call solid, and witniu the very heart of the hills,' which we are taught are everlasting. A seed—fit toy for a babe—contains the'gaarled oak,, the monarch of the forest; the'persecuted caterpillar lapses into the tsrysalis to flutter forth,- whfen its time shaH have come, as the golden butterfly; and in tho stony confines of *ae«ft*th despised charcoal, and quartz sand— •dust—which wben out of place is called " dirt" —are transformed into the fiery ruby or the flashing diamond. _ Theetier-recurring changes seem to endow the •flinty rocks with a life-like existence, their %&- ginning was amidst broil and trouble, tkefo life •was one constant change &,ud their e63 the prorafee of a new or more gloriduii frorld. Though *«Q cannot mark and record tke rate of progress of the changes, the;> j^ 'continually goiDg on. They work, appare^'ijry, j n fchree ways :— First, the subs'tPJjfte of a rock changes its structure and composition without any addition or loss, as cold hammemt iron chauges its structure under the blows of tfeo blacksmith's hammer; or Secondly, there are additionu which white comfcininK -with the rocks, increase their bulk, or else re;p[ac<» a portion of their componentjrjarts ; or Thirdly, parts of the original substance of a rock "are lo3t or removed. In the simple minerals forming \\& component parts of rocks tho changes are most strikingly noted. As for instance, old crystals ai-e! replaced by a new substance without chatiging tbeirform, by cavities opening inthciriritfetiors, which in time becefiie lined with ccyst&a belongnng to a different substauce thkn 'tihat forming i>he parent crystal. Such a e&tubinabion of old crystal-forms enclosing crystals of a new formation are called pseud^metamorphoses, and are of the utmost vstee in determining the age of a *ock, as is se# apparent.. This pseudo-meta-morphosis is often seen in quartz, and from winch it derives its name of " drusy." Cherßistcy shows that the minerals aad some •ot thei;? substances can ha dissolved in water in •differing proportions, and that acids and alkalies, aofc iv a well known manner upon minerals. It has also been ascertained that substances dlxn"cult of solution push aside and replace those of greater solubility. Experimental "chemistry further teaches how a substance t inay be completely changed ,when brought under the influence of any of the known chemical factors* One of the mobt interesting of these esperianents was made by Danbree, at Strfis'b'Urg, by •which clear transparent glass was metamor\phosed or changed into a crystalline mass (woliastonite) of a threadlike stucture in which beautiful quartz crystals, arranged singly or in clusters, pyrox'd&e crystals, and grains of Ma* par were plainly visible. This experiment, &1> though a simple one, is of the greatest importance to science, und consists in heating water to 400deg Celsius (about 740deg gaar) :under a. pressure powerful enough to prevent -the water from boiling,'though heated to a white tow. Not only did this expatteient oucceed in Ranging, m a comparatively short time, the' hardest obsidian lava into crystalline -fcacttyte, but by adding an alkali to the «W8» felspar was produced from clay (kaolin*). Such experiments prove 'much more than volumes of speculative deductions, and though Nature in her labdritory may not follow Danbree s experiment* yet the conversion of a hard Tock into a. -s^t one, and of a soft into a hard on ®» m wbjiofc conversions their very structure and Bubsfet Qce ara completely changed from amorphous to crystalline, and vies versa by tiia same oceans and process, has baen proved to be P<ys>.ole by plain tests, . j.t is well known that all spring water discharged on the surface is charged with earthy particles. It may be accepted therefore that water with its load travels from one place to another in the rocks, and that part $f this load is deposited in a certain spot, in accordance with some chemical or physical law, and also chat in accordance with the sande law it t»k% something else away. The water Vo this manner acts someS"°« h. ke a Parceia '^fc > opening a communicaexoJW between thedifferent parts of the rooks,, wh^ are either difficult to dissolve or else too h^ vy to travel _ Iq labouc the water is J>:sslst;ea by gases which circulate in the rocKS id a aimilar manner, and the gases and the wacer consequently come into contact with flOf 10 j other, exchanging some of their properties 9 Jring the contact or forming entirely new ones. Besides water and gases acfciug upon the solid rock, and consequently rearranging their structure and changing their substance, magnetism aad chemical affiuity are secretly active, and keenly watching that the proper atoms are fitted and joined together and properly arranged. Hence the beautiful clusters of quartz crystals or particles of gypsum, when once one has been formed in felspar, mate with other atoms until they form a full-grown crystal. Iv the artificial experiment above referred to exactly the same thing is goiug on. The liberated siliceous parts of the glass, like obsidian, congregate in one spot, uniting in one beautiful crystal after another, and without the material acted upon becoming soft or liquid, and the component parts of the flinty obsidian change their places in order to form a felspathic crystal. While this process is going on the bulk of the substance ■ acted upon increases enormously in the artificial test just the same as in Nature, and many gradual upheavals can be traced to this cause. Some geologists hold that the European Alps owe their present elevation to this cause. Metamorphism is so general an appearance in the rocks that strictly speaking all known rocks are metamorphosed. If water, percolating through sand, dissolves the lime of the shells in the sand and depositing it again when the water is overcharged with it, binding the grains of sand together, then the sand has been changed. Again, if r.u'n falling upou lava molts some of its parts, and running through the liolps in the, lava, leaving in them part of tho solMs of th- 3 solution. th» lavi Iris undergone a raf>tamo» *. phosis. Sci>nci», however, draws a lino in spep ing of ni^taraorphosed rocks at the point wh> »re the change of the component parts has gom /go ixe as to maktt it impossible to f«ay what the rocks originally were like. Nob oulv ar» the rocks subject to sneh hanger, but even the metals which they f §on-

tain are acted upon by the chemib&l factors. Metals can be dissipated IWb gases ; they can be qasol^Ju gcfds» M sugar ia water V^ i can be divided Into atoms so fine as to be fya- 1 >rißfole Vd'fche' n^St eye. But there, are, , in Nature what are known as re-agents, wliich.c^l--lect the finest atoms of the satne material from the gases and the water intb grains and oiyßtab, depositing them at .su|t'aKe places. The oxide df iron 'and kft Other iron' ores are found so frequently as pseudo-metamorphoses that thdrte ts no longer any room for doubt about !their formation and origin. Exactly the same is the case with the ores of copper, silver, lead, zinc, manganese, and many less known metalb, which have all been found to reproduce the finest and most delicate parts of plants and animals, which buried. and decaying in the rock left the moulds of their forms in its stony grasp. So perfect are many of these metallic fossils that for accuracy and completeness they far excel the best preserved specimens of the original, These wonderful metallic fossil— fofmS thus not only supply missing sentences sii the history of the rocks, but they prove clearly how the metals have been deposited. Some of the ftt&s't highly metamorphosed rooks are t Ttachyte, granite, gneiss, the schist rocks ftfc'fi their subdivisions, and as these rocks ia?e also the richest in metals they are of the greatest importance to miners.