236 



KNOWLEDGE. 



[October 2, 1899. 



which are often imitated in artificial forms, have thus 

 originated in the great earth-mill. On a milder scale, but 

 none the less convincing, we have the red Carboniferous 

 marble of Cork in our own islands, which has assumed a 

 sort of flow-structure during the overfolding of the district. 

 Knots of the original fossOiferous rock are freely left in it, 

 indicating the former structure of the mass. 



Wherever, again, soft beds and brittle ones alternate in a 

 stratified series, the less pliable beds tend to become broken 

 up, while the yielding layers flow into their interstices. 

 A false effect of fragmentation is thus produced, which is 

 often intensified by the rounding of the included blocks. 

 Such fragmental rocks of secondary origin have been styled 

 " crush-conulomerates," and are of course nothing more 

 than breccias in which flow has taken place. The whole 

 mass may be bounded at its margins by great fault-planes ; 

 but at the same time every block, and almost every layer, 

 is faulted on a small scale against its neighbours. 



Excellent examples of this form of brecciation may be 

 seen where limestones and shales, in alternate beds, have 

 become folded and compressed. Wherever a means of 

 escape lay open, wherever the material could give way and 

 flow, the shales have oozed out, and the limestones, which 

 could not thus be elongated, have been at once broken 

 into blocks. As the process goes on, these blocks become 

 rounded until they resemble pebbles. t It is only here and 

 there, by tracing the original bedding, that we are able 

 mentally to reconstruct the series. 



Such movements naturally have been noticed in rocks 

 ■where true pebbles already exist. The Permian con- 

 glomerates of Worcestershire show many signs of the 

 grinding of one hard pebble against the other, and the 

 resulting striation, or " slickensiding," is so complete that 

 it has been adduced as a proof of glacial action.^ The 

 quartzite pebbles, often two feet long, in the conglomerate 

 of Cushendun, in Antrim, have been broken through again 

 and again, the parts being faulted and then re-cemented 

 at the contact. The limestone pebbles, again, of the 

 " Nagelfluh " of northern Switzerland have actually been 

 forced into one another, solution taking place at the same 

 time, so that a large pebble may bear indentations from 

 several of its neighbours, like the impress of a potter's 

 thumb. Had the Alpine earth-movements continued, 

 these massive Swiss conglomerates might have been 

 reduced to crushed and unrecognisable calcareous shales. 



If these things are done in the upper layers of the crust, 

 and among rocks on the mere outskirts of a mountain 

 range, we can form some conception of the changes that 

 go on in the lower levels of the mill. The central axes of 

 our mountains, moreover, where denudation has had full 

 play, show us the crushed and crumpled rocks brought up 

 by folding from below. In certain regions, again, such as 

 Scandinavia and north-west Scotland, mountain ranges 

 have been worn down to the roots, have been laid bare in 

 horizontal section ; and here we come into touch with 

 some of the more intimate secrets of the crust. 



Prof. LapworthS has recently dealt so clearly and 

 succinctly with the whole question of metamorphic rocks, 

 that one feels inclined to refer the reader at once to his 



* See the full description by Mr. Gr. W. Lamplugli. " The Crush- 

 Conglomerates of the Isle of Man," Quart. Journ. Geol. Soc, 

 Tol. LI. ( 1895), p. 563. 



t Good illustrations of these processes are given in a paper by 

 Messrs. Gardiner and ReTnolils on " The Portraiue Inlier," Quart. 

 Journ. Geol. Soc, Tol. LIII. (1897), pp. 528 and 532. 



* See remarks bv Mr. AV. Wickhani King, Quart. Journ. Geol. Soc, 

 Vol. LV. (1899), p. 128. 



§ " An Intermediate Text-Book of Geology " (1899). pp. 113— lio. 



historical and critical review. As a master in the work of 

 unravelling metamorphic areas in the field, he perceives 

 the difiiculties of the student, and appreciates those cases 

 which are susceptible of various explanations. Broad areas 

 of gneiss occur, in which the foliation may be an original 

 structure; the minerals in such cases, even while consoli- 

 dating, may have been dragged out into lenticular forms 

 by the flow of the viscid mass. But, in other cases, there 

 are distinct signs of crushing ; the flow is that of a 

 brecciated material, and the knots and eyes represent those 

 portions of the original mass that have escaped being 

 ground to powder. 



Prof. Lapwortli Las proposed the name " mylonite," 

 from auAojv, a mill, for rocks in which the finer materials 

 '' have been sheared, dragged, and ground between the jawa 

 of the gliding-planes."* The compact basis of such rocks 

 is in truth composed of the flour of the great earth-mill. 

 In microscopic sections cut from them, we see reproduced 

 all the features that can be studied on a large scale in the 

 neighbourhood of broken folds and faults. Matters are 

 often complicated by the development of new crystalline 

 materials along the planes of flow and foliation ; but the 

 conspicuous minerals in mylonites are commonly true 

 residual eyes. In the example here chosen (Fig. 1), the 

 gneissic structure is not readily apparent until polarised 

 light is employed. Then the lumps of quartz and felspar 

 are seen to be embedded in a fine rock-flour, which results 

 from their abrasion. Whole flakes of quartz have been 

 removed by shearing, and are accompanied, comet-like, 

 by tails and streams of their own dust. The rock was 

 originally a granite, almost free from mica ; but it has 

 begun to flow under earth-pressure, and its hardest 

 minerals have been ground to powder along the shearing 

 planes that traverse the whole mass. 



By microscopic evidence, we are fairly able to discriminate 

 between original and secondary flow. Quartzites, for 

 instance, may be found, in which a flaky structure has 

 been set up, and in which the larger grains are all lentictilar. 

 Such rocks, however, are not likely to have emerged as 

 fluid lava from a caldron. The microscope shows how the 

 lenticles are mere relics of larger granules, the opposite 

 sides of which have been ground ofl', while a dust of quartz- 

 fragments streams away along the planes of movement, 

 ^lany grains are pulverised over all their surfaces, and we 

 may conclude that others have disappeared altogether with 

 the growth of the mylonitic ground.! 



Rocks composed of more perishable or softer materials 

 naturally sufi'er in still greater degree from the earth-mill, 

 (labbros go to pieces, the augites becoming dragged out 

 as bands of brown-grey powder, and the basic felspars 

 forming a compact and almost porcellanous ground.; In 

 a great complex series of rocks, subjected throughout to 

 pressure-metamorphism, it may often become impossible 

 to recognise the character and relations of the original 

 constituent masses. 



We have passed from the inspection of fault-rock and 

 breccias in the field to a consideration of rocks that may 

 be regarded as regional breccias, and not as mere local 

 accidents along planes of fracture. Down to their minutest 

 structural details, these schists and gneisses bear witness 

 to the great earth-mill. When we return from the 

 laboratory to a review of their larger features, we see 

 how one whole series of rocks may play the part of a 



* Op. cit,, p. 14t. 



t See Sollas, '■ On the Structure and Origin of the Quartzite Eocis 

 in the Neighbourhood of Dublin," Sci. Proc. R. Dub. Soc, Vol. VII. 

 (1892), p. 169. 



J The rock of Penig, in Saxony, is a fine esample. 



