l82 



NATURE 



[June 23, 1892 



this way, now in that ? So it becomes necessary to correlate 

 our observations, to frame hypotheses, and open out new lines 

 of inquiry. 



So far as we know, water, pressure, heat, are the main agents 

 in producing change in rocks, after the latter have been once 

 deposited or solidified. In most cases it is not easy to insulate 

 perfectly the effect of each agent, for probably every rock, which 

 has undergone important changes, has been to some extent 

 affected by all of them. Still many examples can be found, in 

 which the influence of one has predominated greatly over that 

 of the other two. For instance, it is now agreed that the struc- 

 ture of a slate is the result of pressure, though this probably pro- 

 duced a slight rise of temperature, and the rock is not likely to 

 have been perfectly dry. Again, when a clay has been con- 

 verted into an assemblage of crystalline silicates in the vicinity 

 of an intrusive mass of granite, this is mainly the effect of heat, 

 though the pressure cannot have been inconsiderable, and the 

 presence of water is almost certainly essential. 



Thus, in one series of examples, properly selected to illustrate 

 the slaty rocks, we can watch the development of new minerals. 

 We can observe which of these are readily produced and quickly 

 attain to a considerable size, which are more slowly formed, or 

 seem incapable, even if common, of much enlargement. We 

 are thus led by inductive processes to conclusions as to the effects 

 of pressure in the development of minerals in a mass of materials 

 of a particular composition. In another series of rocks which 

 has been affected by the heat of intrusive masses, we can watch 

 the gradual growth of new constituents, as we proceed inward 

 towards the originally heated mass, till we have passed from 

 clay or slate to a crystalline aggregate of minerals, such as 

 quartz, micas, andalusite, staurolite, and garnet. Similar effects 

 may be noted in other kinds of sedimentary rock. Changes also 

 are produced mainly by the action of water, but on this I need 

 not enlarge. 



Again, as another line of inquiry, the effects produced on 

 igneous rocks by the same agents must be studied. Here the 

 results which are more or less directly due to the action of water 

 are often highly interesting, but as these are only indirectly con- 

 nected with the main subject of this lecture, I content myself 

 with a passing reference. With igneous rocks the effects of heat 

 seem generally less important than with sedimentary ; probably 

 because the mineral constituents of the former are usually in a 

 more stable condition than those of the latter, so that these also 

 need only be mentioned ; but the effects of pressure in some 

 cases, especially with the more coarsely crystalline igneous 

 rocks, are highly interesting and significant. 



In a region such as the Scotch Highlands or the European 

 Alps the rocks, in the process of mountain-making, have been 

 obviously subjected, perhaps at more than one epoch, to tre- 

 mendous pressures. The effect of these appears to have been 

 sometimes a direct, sometimes a shearing fracture ; that is to say, 

 a mineral or rock, in the one case, has been crushed, as in a 

 press, in the other, during the process of powdering it has been 

 dragged or trailed out, with a movement somewhat similar to 

 that of a viscous substance. As an example, let us take the 

 effects produced in a granite by crushing. The grains of quartz 

 are broken up ; the crystals of felspar are first cracked and then 

 reduced to powder ; the mica flakes are bent, riven, and tattered. 

 By pressure also the solvent power of water, already present in 

 the rock, is increased ; by the crushing its access to every frag- 

 ment and its subsequent percolation are facilitated. Thus the 

 black mica is often altered in various ways ; the felspar dust is 

 changed into white mica and chalcedonic quartz ; the constituents 

 are reduced in size and tend to assume a roughly parallel order ; 

 the mineral character and structure have been alike changed ; a 

 massive rock has been replaced by a foliated one ; a coarse 

 granite by a fine-grained quartzose or micaceous schist. This 

 change can be demonstrated at every stage ; it suggests that 

 many foliated rocks — many gneisses and crystalline schists — may 

 be igneous rocks of which both the mineral character and the 

 structure have been modified by pressure. 



We may presently see how far this inference can be justifiably 

 extended, but, as a first step, the effect of pressure on one of the 

 more basic igneous rocks must be considered. Let us take as 

 an example a coarse-grained variety of the rock, which is familiar 

 to us as basalt. It consists of a felspar, different from that of 

 granite, of augite, of some iron oxide, and perhaps of olivine. 

 In studying this rock we are confronted by greater difficulties, 

 for, of the two dominant minerals, the felspar is rather less stable 

 than that which occurs in granite, and the augite passes readily 



NO. II 8 2, VOL. 46] 



into hornblende. Thus, when the latter change occurs we are 

 at first unable to determine whether it is due to pressure or to 

 some other agent. Some petrologists, I believe, would not 

 hesitate to appeal to the presence of hornblende in a rock such 

 as we are considering as a proof that it had been modified by 

 pressure. With this opinion I cannot agree. On examination 

 of the numerous instances in which we are convinced that the 

 hornblende is not an original constituent but has replaced augite, 

 we notice that the former mineral is not constant in its characters. 

 It may be granular in form ; it may assume its usual crystalline 

 shape ; it may be more or less bladed or needle-like. Have 

 these differences, we ask, any significance? In order to 

 answer the question, specimens of hornblendic rocks must 

 be sought in regions which obviously have been subjected 

 to tremendous pressure, as is testified by the fact that every 

 other rock has been more or less crushed or rolled out ; others 

 must be obtained from regions where the associated masses 

 exhibit no signs of extraordinary disturbance, even though they 

 may be more brittle than the subject of our study. In the former 

 case the change may be reasonably attributed to pressure, in the 

 latter it must be due to some other cause. Are hornblendic rocks 

 from the one region similar in structure to those from the other ? 

 By no means. Where no evidence can be offered in favour of 

 pressure, there the hornblende either retains wholly or almost 

 wholly the outline of the mineral which it has replaced, or else 

 assumes its normal prismatic form ; but where an appeal to pres- 

 sure seems justifiable, we find that the hornblende appears as un- 

 usually elongated prisms, blades, or even needles, and the struc- 

 tures of the rock as a whole can be readily recognized by a 

 practised eye. The evidence for the latter statement is yet un- 

 published, but it will, I hope, appear before long. 



So our investigations have led us thus far : that, in sediment- 

 ary rocks, in the presence of water, certain changes are mainly 

 produced by heat, and certain by pressure. In the latter case^ 

 however, the new minerals, though very numerous individually, 

 are generally minute ; the longest diameter being seldom so much 

 as one-hundredth of an inch. Even where this rule is broken, it 

 is only by minerals which are proved by other experiments to be 

 so readily developed that their presence on a large scale has no 

 real significance. The rule holds also to some extent in the case 

 of crystalline schists produced by the crushing of crystalline rocks,, 

 markedly in the case of those derived from granites and rocks of 

 similar composition, but less conspicuously in those which were 

 originally augitic or hornblendic. Though even here, where the 

 decreased size of the minerals is less uniformly marked, new and 

 distinctive structures are assumed. 



I have spoken only of two or three common types of rocks, 

 but it would be easy, did time permit, to support the principles 

 enumerated, by quoting from a great variety of examples. There 

 are, I believe, few, if any, important kinds of rock which have 

 not been examined, and it appears to me demonstrated that, 

 while pressure is a most important agent of change,^ while many 

 schists maybe regarded as resulting from it, a considerable group- 

 remains, which are separated from the others by a very wide 

 chasm, and this can only be jumped by deserting reason and 

 trusting hypothesis. 



In this last group of rocks (supposing no disturbances pro- 

 duced by subsequent pressure, for which, however, we can 

 generally make allowance) the constituent minerals are com- 

 monly fairly large — say from about one-fiftieth of an inch up- 

 wards in diameter. Very many of these rocks, when studied 

 in the field, exhibit every indication of a sedimentary origin. 

 Though as a rule no original constituent grain can be cer- 

 tainly determined, though they are now crystalline, yet 

 their general structure and association are inexplicable on any 

 other supposition. They bear some resemblance to the sedi- 

 ments which have been altered by contact metamorpbism, though 

 they present different characters. These, moreover, remain in- 

 variable through considerable thicknesses of rock and over wide 

 areas. The alteration is regional, not local, so that such rocks can- 

 not be regarded as cases of simple contact metamorpbism, even 

 though heat may be suspected of having been an important agent 

 in producing the change. But to another large series, including 

 many of the rocks commonly called gneisses, the sedimentary 

 origin is less easily attributed. Not a few of these in mineral com- 



I It is probable that some changes of importance are produced in rocks by 

 long-continued and repeated pressures, which are insufficient to give rise to 

 crushing ; but these 1 have passed by, because, as it seems to me, further 

 evidence is needed before we can diagnose, witli any certainty, the results of 

 this particular disturbing cause. 



