4i8 



NA TURE 



\_AIairh 3, 1887 



continual change, new adjustments of the molecular struc- 

 ture of the rocks are at once necessitated and brought 

 about. 



In attempting to reason as to the original conditions 

 under which a rock-mass must have been formed, it is of 

 ^gfeat importance to avoid those sources of error which 

 •exist in rocks that have undergone much secondary altera- 

 tion. Such rocks abound in, tliough they are not neces- 

 sarily confined to, the older geological formations ; and 

 it is among the younger and fresher rocks, therefore, that 

 vvemiy most hopefully seek the key to nnny petrological 

 problems. 



If, for example, we concentrate our attention upon the 

 more recent and less altered igneous rocks, it becomes 

 ■clear that the degree of crystallisation displayed by them 

 has depended on the slowness with which consolidation 

 has taken place, and that this has in turn been deter- 

 mined by the depth from the surface at which they have 

 been formed. In this way, by the study of igneous rock- 

 masses in Scotland and in Hungary, I was able to show 

 that there is a perfect gradation from highly crystalline 

 rocks — granites, diorites, and gabbros — into the ordinary 

 volcanic types — rhyolites, andesites, and basalts, respect- 

 ively — and from the latter into the various kinds of vol- 

 canic glass. These conclusions have been confirmed by 

 subsequent investigations like those of Hague and Iddings 

 in the Comstock region, and of Lotto in Elba. Further 

 and more recei)t researches have enabled me to show 

 that certain types of structure have been determined in 

 rocks, according to the more or less perfect absence of 

 all movement within them during their consolidation. 



Very remarkable, indeed, are the internal changes 

 which take place in rock-masses when they are submitted 

 to those powerful stresses which result from the move- 

 ments that occur during mountain-making ; and the full 

 explanation of these is perhaps the most difficult problem 

 which still confronts the geologist. 



It was long ago asserted by Scrope and Darwin that 

 the solid rock-masses of the globe, under such conditions 

 as these, must have actuallyy/^^jctv/, like the viscous lavas 

 of the rhyolitic series. These geologists were even able 

 to show that the separation and disposition of the crystal- 

 line elements in such lavas present the closest analogy 

 with what is seen in the crystalline schists and gneisses 

 of greatly disturbed areas. 



Since these early researches, which were principally 

 based on the study of rocks in the field, aided only by 

 the pocket-lens, three classes of researches have served 

 to deepen our insight into the methods by which the 

 schistose and gneissose rocks must have been produced. 



In the first place, the experiments of MM. Tresca and 

 Ddubred have shown that solid matter under enormous 

 pressure behaves like a viscous substance, its whole in- 

 ternal structure exhibiting evidence of the flowing move- 

 ments to which it has been subjected. 



In the second place, the studies of M. Spring have 

 established the fact that both paramorphic change and 

 direct chemical reaction may result from simple pressure. 

 Thus the unstable monoclinic form of sulphur, by a 

 pressure of 5000 atmospheres, was at ordinary tempera- 

 tures converted instantly into the stable rhombic form, 

 a transformation accompanied by change of density 

 and of many other physical properties. Still more striking 

 is the case of the unstable, yellow, rhombic, mercuric- 

 iodide, which, by simple rubbing with a hard substance, 

 passes into its stable, red, tetragonal allomorph. It is 

 instructive to notice that the same change in both 

 instances appears to take place "spontaneously" after 

 a sufficient interval ol time ; or, in other words, small 

 variations in temperature, pressure, and other surround- 

 ing conditions are capable, if sufficient time be allowed, 

 of bringing about the same result as more intense 

 pressure applied suddenly. That the similar paramorphic 

 change of pyroxene into hornblende, which is so frequently 



exemplified in the earth's crust, is sometimes the result 

 of intense pressure, and at other times follows from 

 the repeated slight alteration of conditions during long 

 periods of time, we have, I believe, abundant evidence. 



But the experiments of M. Spring that prove that 

 chemical reactions can result directly from pressure 

 are of even greater interest to the geologist. By submit- 

 ting mixed powders to intense pressure, he succeeded in 

 producing metallic alloys and various binary compounds, 

 and also in bringing about double decomposition between 

 many sahs. That similar reactions between the com- 

 plicated silicates which form the minerals of rocks have 

 resulted from the enormous pressures to which they have 

 been subjected, we have the most ample proof. Thus in 

 rocks where such pressure has just begun to act, such as 

 the " tlaser-gabbros," wherever the unstable olivine is 

 in contact with the almost equally unstable anorthite, 

 chemical reactions have been set up by the pressure, and 

 these have resulted in the formation of zones of enstatite 

 and anthophyllite, hornblende and biotite, which have 

 been so well described by Tornebohm, Bonney, Adams, 

 and Williams. Provided with the clue supplied by these 

 results, we find little difficulty in going one step further. 

 When the pressure has been still more intense, as in 

 mountain-making movements, reactions are set up among 

 all the minerals of the rock-mass ; the elements of which 

 it is composed, set free from their old engagements, enter 

 into new alliances, and the result is the formation of a 

 completely new set of crystallised minerals. 



The third class of researches, destined, as I believe, to 

 remove our difficulties in e.xplaining the origin of the 

 schistose and gneissose rocks, are those already alluded to 

 as having been undertaken with the microscope. As yet 

 the details of such changes have only been explained in the 

 case of some of the simpler examples ; but I am convinced 

 that the persevering application of the same methods in 

 the field and the laboratory will result in the removal 

 of difficulties that now seem to be absolutely insur- 

 mountable. 



Some observers in this country have been led to infer 

 that the recrystallisation of rock-masses under pressure 

 has in all cases been preceded by their pulverisation. Of 

 this, I cdnfess that I can find no evidence. That near 

 great faults of all kinds, this reduction of rocks to powder 

 does take place, we find abundant proof ; but the evidence 

 also points to the conclusion that such rocic-crushing, as 

 distinct from roc/:-_flinuiiig, is in c\'ery case local and 

 e-cceptional. 



There is another and totally different series of changes 

 which takes place in rocks, when, brought near to the 

 surface by denudation, they are exposed to the action of 

 water, oxygen, carbonic acid, and other atmospheric 

 agents. The breaking-up of the alkaline silicates and 

 the deposition of secondary silica, the formation of the 

 zeolites, the epidotes, the chlorites, and serpentine, the 

 resolution of crystallised minerals into the isotropic 

 mixtures, and the recrystallisation of these in new forms, 

 all offer problems of the highest interest to the geologist. 



I may venture, in drawing these remarks to a close, to 

 indicate another point of analogy between the three 

 natural-history sciences. It is found in the circumstance 

 that experimental verifications of our conclusions are 

 often difficult, if not actually impossible. 



We must be content to reason from the proved varia- 

 bility of the existing forms of plants and animals as to the 

 possibility of the production in time of new species. And 

 in the same way, with our limited command of heat, 

 pressure, and especially of time, we can scarcely hope to 

 originate the exact counterparts of the various minerals 

 and rocks of our earth's crust. 



We may nevertheless point with satisfaction to what, 

 in spite of such difficulty, has already been accomplished 

 in this interesting field of research. The honour of having 



