September 23, 1915] 



NATURE 



lOI 



stage the unmelted sediments exhibit plasticity and 

 become intensely contorted. The softening, in fact, 

 induces flow. There is here no crushing or myloni- 

 tisation, but rather a viscid running' of constituents, 

 some on the verge of fusion, some, I venture to 

 think, actually fused. Such rapidly repeated and intri- 

 cate folding is most intense when lit par lit injection 

 has set in, and when the whole composite mass has 

 become weak and plastic. The presence of confined 

 water in aiding this plasticity must on no account be 

 overlooked. 



It may be well to illustrate this contention by one 

 or two concrete instances from districts not remote 

 from us at the present time. The noble cliffs of 

 Minaun in Achill Island have been worn by the Atlan- 

 tic from a mass of evenly-bedded quartzites of Dal- 

 riadian age. These are invaded by veins of a very 

 coarse red granite, the main mass of which lies below 

 the present sea-level." The edges of the strata appear 

 fairly horizontal on the cliff-face; but contortion sets 

 in towards the base, and the hard resisting rock has 

 here ^^ " undergone intense crumpling and overfolding, 

 such as one meets with on a large scale in mountain 

 ranges, and this contorted flow seems entirely due to 

 j the yielding that has taken place in the region of 

 I heating." The large size of the constituent crystals 

 i of the granite indicates that the surrounding rock was 

 I still maintained at a high temperature. 

 ; South of Foxford, again, in the county of Mayo, 

 I the granite of Slieve Gamph invades a series of mica- 

 schists and quartzites. The margin is cut, as usual, 

 by veins that filled the cracks both of the main granite 

 and the metamorphosed sediments. These sediments 

 have become, prior to the shattering, crumpled and 

 overfolded along the contact-region, and the section 

 upon the glaciated slope resembles that of a fluidal 

 rhvolite on a highly magnified scale. 



The wonderful contortion of the composite mass that 

 forms the north end of the Ox Mountains (Slieve 

 Gamph) in the county of Leitrim gives a similar 

 impression of viscid flow. The melting of a single 

 constituent of the invaded schists, which here include 

 amphibolites, would enable them to yield in response 

 to the pressures that were forcing the granite magma 

 in thin sheets between them. Their metamorphism 

 is thermal, and the forces that have produced the 

 crumplings are not those of shearing acting on a solid 

 mass, but may have operated from a distance hydro- 

 statically through the magma. 



Again, where limestones occur near granite con- 

 tacts amid a series of various sedimentary types, they 

 display folded structures in an altogether exceptional 

 degree. Silicates have developed along their bedding- 

 planes, but these have become contorted and rolled 

 upon one another as metamorphism reached Its maxi- 

 mum stage. At Maam Cross and Oughterard, in the 

 county of Galway, along the margin of the great 

 anite mass that stretches thence southward to the 

 M. these flow-structures are conspicuous on weathered 

 surfaces. 



The main object of the foregoing discussion is to 

 point out that the Huttonian cycle. In which thermal 

 changes play so large a part. Implies a serious weaken- 

 ing of the crust as magmas advance Into It from 

 below. The extensive metamorphism of the pre- 

 Cambrlan strata, which amounts to a distinctive 

 feature, must, I think, be attributed, not to special 

 Intensity of tangential pressures In early times, but to 

 frequency of igneous attack. Much of the crumpline 

 of our schists may result from Hutton's "softening," 

 fhe pressure being supplied from superincumbent 

 masses, or even hydrostatically, and the flow occurring 



11 Proc. Cenl. Assoc, vol. xxiv. (1013). Plate 17. 



I- G. A. J. Cole, " IllM«trations of Composite Gneisse* and Amphibolites 

 !n N. W. Ireland," C. R. Con^ri>s gM. ittternat., Canada (1913), p. 312. 



laterally, or vertically downwards, towards regions 

 where destruction by absorption was going on. The 

 features seen during the falling in of the walls of the 

 lava-lake of Kilauea in Hawaii afford some idea of 

 what takes place in zones of melting within the crust. 

 Under such conditions in early pre-Cambrian times, 

 even the surface-rocks must have fallen in at some 

 points and have been replaced by igneous ex- 

 trusions. Isostatic adjustments must have been 

 very frequently disturbed. Folding of rocks, as a 

 phenomenon of lateral surge and flow, must have 

 made itself freely felt at the earth's surface. It is 

 safe to assert that such conditions have not been 

 repeated on a broad scale at any geological period 

 subsequent to the spread of the Olenellus-fauna. Geo- 

 chemlcal evolution, however, may have surprises still in 

 store, and, in spite of long tradition, we are dis- 

 inclined nowadays to rely too strongly on arguments 

 based upon the sanctity of human life. 



Possible Breaks in the Slow Continuity of Earth- 

 movement. 

 I. The Mountain-building Stage. 



Even with the thickened sedimentary crust beneath 

 us, and the confidence inspired by our limited experi- 

 ence of the earth, we may ask if subterranean changes 

 may not still result in catastrophes at the surface. 



What, in fact, is likely to occur if a mountain-build- 

 ing episode again sets in? Such episodes, affecting 

 very wide areas, have undoubtedly recurred in the 

 earth's history. We do not know if they are rhythmic ; 

 we do not know if they represent a pulsation, decreas- 

 ing in intensity, inherited from the stars and hampered 

 by increasing friction ; we do not know if they record 

 internal chemical changes, which have no climax, 

 because they are neither cyclic nor involutionary, but 

 evolutionary. The mid-Huronian chains, now worn 

 down and supplying such valuable horizontal sections, 

 were evidently of great extent ; but we cannot say that 

 they were vaster than those of later times. 



The phenomena accompanying the growth of a 

 single chain in the Cainozoic era give us, at any rate, 

 ample food for thought. Though the narrow cross- 

 section of the core of such a chain limits our field 

 of observation, the same inpressings of igneous mate- 

 rial, and the same features of rock-weakening and 

 rock-destruction, may be observed as in the immense 

 basal sections exposed in the Archaean platforms. The 

 progress of geological time has not diminished the 

 activity in the depths. The granodiorlte of western 

 Montana,*' for instance, which intruded during an 

 uplift in early Eocene times, has attacked the Algon- 

 kian sediments of the district, producing phenomena of 

 stoping and assimilation in the true " Laurentian " 

 style. 



In the western and central Alps, again, the absence 

 of any fossiliferous strata older than the Carboniferous 

 arouses some surprise, until we find that many of 

 the granitic intrusions are of late Carboniferous age. 

 The crystalline schists west of Caslav in Bohemia and 

 in the Eisengebirge are now attributed by Hinter- 

 lechner and von John '■* to the metamorphism of Ordo- 

 vlclan strata by younger granite, which intruded in 

 post-Devonian and probably in Carboniferous times. 

 Much of the gneiss and granite of the Black Forest 

 and the Vosges is now, moreover, removed from the 

 Archaean, and is shown to be associated with the 

 Armorican movements." These vast intrusive 



!•' J. Barrell, " Marvsville district, Montana : a study of ieneous intrusion 

 and "contact metamorphism " l/.S. Geo/. Sntrry, Prof. Paper 57 (iqo?), 

 and W. H. Emmons and F. C. Calkins, " Phillipsburg Quadrangle," ibid.. 

 Paper 78 ^1913). 



H Verhandl. k. k. Reichsanstalt, 1910, p. 337, and Jahrb., ibid., vol. lix. 

 (1009). p. 127. 



l» P. Kessler, "Die Entstehung von Schwarzwald und Vogesen," 

 Jahrtsberichte Oberrhtin, gcol. Vereines, vol. iv. (1914), p. 31. 



NO. 



2395, VOL. 96] 



