188 



KNOWLEDGE. 



[August 1, 1899. 



reach down into the volcanic core. The elaborate proofs, 

 moreover, afforded by the mine-shafts of Nevada,* 

 destroyed any belief in America as to the structural 

 distinction of Cainozoic and older igneous rocks. Highly 

 crystalline rocks, regarded previously as ancient, were 

 found to be in continuity, in sections three thousand feet 

 in depth, with the lavas that had been described as 

 Cainozoic. 



Finally, the authors of rock-classifications based on 

 geological age retreated step by step, as Von Buch and the 

 Wernerians had done before them ; and we may now seek 

 to read, without opposition, the history of a modern 

 volcanic caldron in one of our old deep-seated masses. 



The dykes and veins sent off at the margins of such masses 

 seem natural enough ; the rocks have cracked and yielded 

 round about the igneous invader. But the actual position of 

 this invader raises many difficult problems ; and the huge 

 mass of the caldron or reservoir is often astounding, 

 compared with anything that we see in volcanic action at 

 the surface. When, however, we consider the extent of 

 what Prof. Judd has called volcanic provinces — areas 

 over which similar volcanic rocks are simultaneously 

 erupted — we may well see that one caldron may have to 

 serve vents scattered over thousands of square miles. 

 Many caldrons, moreover, may remain unconnected with 

 the surface, and may fulfil their period of activity in 

 merely attacking their surroundings. 



Whether connected with a true vent or not, how does 

 the reservoir arise '.' Its frequent nearness to the surface, 

 considering the great thickness of the solid crust, makes 

 it improbable that it remained as a liquid mass from the 

 first consolidation of the globe. Evidence of the intrusion 

 of the liquid, moreover, is again and again forthcoming ; 

 yet how was space found for it in the tightly packed 

 material of the crust ? 



A tempting theory, current from Hutton's time down to 

 about 1860, was based on the assumption that sedimentary 

 or other rocks could be metamorphosed into a highly 

 crystalline state in situ, while still retaining at their 

 margins traces of their former structure.! What may 

 thus be called progressive metamorphism provided an easy 

 escape from the difficulties of an intrusive theory. The 

 granite or gabbro merely represented the mass that was 

 there before crystallisation set in. A partial melting 

 might then account for the offshoots at its margins. Case 

 after case, however, broke down upon investigation ; and 

 the modern tendency has been to refer one metamorphic 

 rock after another to an igneous origin, rather than to 

 allow any igneous rock a metamorphic origin. It can 

 hardly be denied that sedimentary materials have from 

 time to time become melted up since their first appearance 

 on the globe ; but this implies a complete change in their 

 character, and the cycle of events may be very rarely 

 traceable. 



Seeing that igneous locks can have little inherent power 

 to force open cavities for themselves, we are apt to fall 

 back on earth-movements to explain the position of our 

 caldrons ; and we may reasonably suppose that rooks 

 which were once in contact may become bent apart during 

 the severities of folding. Such separations take place in 

 the experiments that have been devised to imitate moun- 

 tain-structures ; and, in nature, any molten material would 

 at once be forced into the hollow as it formed. External 

 pressure on this material, and the expansion of the water 

 contained in it, would account for its intrusion. 



It is not surprising, then, that what have been called 



' * Hague and Iddings, £idl. U. S. Oeol. Siirvei/, No. 17 (1885). 

 + Cf. Portlock, " Gcol. Report ou Londonden^, &c.," 1843, p. 507. 



" lacoolites "* occur, where one series of strata has been 

 pushed up in a dome, like a great bubble, over the surface 

 of the underlying beds, the cavity being filled, step by 

 step, by intrusive igneous matter. It is generally believed 

 that the igneous rock, acting under earth-pressures, is itself 

 the cause of the local uplift and of the formation of the 

 dome. Here we have our caldron formed progressively. 

 Any fissures that open above it may lead to volcanic action 

 at the surface. 



Given the caldron, however, can we conceive it to have no 

 action, beyond mere metamorphism, on its surroundings ? 

 Almost every crystalline mass, when worked over in the 

 field, shows signs of local variation. The subject has been 

 recently discussed, in its broad aspects, by Sir A. Geikie,} 

 and absorbs the attention of petrologists in every land. 

 The same rock-mass — our consolidated caldron — may be 

 extremely rich in silica in one part, and may consequently 

 have given rise to granite or quartz -diorite ; while in 

 another part it may be poor in silica and rich in magnesia 

 and iron. Extreme types of igneous material may thus be 

 found in association, and pass by degrees into one another. 

 Where movement has taken place, two kinds of material 

 may have become streaked out together, so that a banded 

 gneissic rock results, truly igneous in origin, but implying 

 differences in constitution in the original magma of the 

 caldron. 



How have these differences been brought about ? Elabo- 

 rate theories have been devised, whereby it is shown that 

 a mixed mass of silicates will naturally tend to divide 

 itself into magmas of varying composition. On the other 

 hand, we have striking examples of igneous masses, 

 extending over thirty or fifty linear miles, in which very 

 little variation can be traced. It must be admitted, 

 however, that in such cases the differentiated portions may 

 lie far away below our feet, or may have been already 

 removed by denudation. 



When we view, in a fortunate locality, the edge of a 

 great igneous mass against a series of stratified rocks, we 

 must be struck with the extent to which blocks of the 

 latter arc picked off and lie scattered through the invader. 

 But these disappear as we retreat from the contact-zone, 

 and sometimes do so with surprising rapidity. Can it be 

 that the caldron has attacked its surroundings for a 

 distance of fifty or sixty feet, but was powerless to effect 

 further destruction ? 



If we examine the included blocks themselves, we find 

 them altered in various degrees ; many of the smaller 

 ones are penetrated throughout by the igneous material, 

 and have become completely crystalline. Shales and slates 

 have thus come to assume the aspect of basic igneous 

 rocks. By intermixture of the materials, and by partial 

 absorption of the sediment in the igneous invader, new 

 rocks have been actually formed. If these fragments have 

 happily survived the extremes of change, how many 

 more may have been completely dissolved in the great 

 caldron '? Is it not probable that vast masses of sediment, 

 penetrated by the insidious magma along all their cracks 

 and crevices, have been cut up into detached portions, and 

 have entirely melted and disappeared ? 



But such absorption must have influenced the chemical 

 composition of the invader. In many cases, the bulk of 

 the latter is so great that little change has been effected 

 in it. In other cases noticeable differences have arisen. 

 Can we not explain many supposed cases of " magmatio 

 differentiation " by such local phenom ena of absorption ? 



* Gr. K. Gilbert, "Geology of the Henry Mountains," 1877, p. 51, 

 &c. ; also "Whitman Cross, " Laecolitic Mountain Orronps," 14t/i Ann. 

 Sep. U.S. Qeol. Stirvei/, p. 236. 



t" Ancient Volcanoes of Great Britain," Vol. I., p. 90, etc. 



