G, Poulctt Scrope — On Lavas, 103 



portions ; the escaping steam can-yiiif? ofT an immenfio amount of 

 caloric. The finer the grain the more readily would tlio struggling 

 vapour be enabled to collect and rise in bubbles, and benco the 

 more vesicular structure of the glassy lavas ; while in those of 

 coarser or more crystalline grain, the expanding steam would be 

 likely to remain longer entangled in the magma, causing it to 

 assume a spongy or loosely granular texture, and to swell up 

 like a mass of dough or paste in an oven ; while tho vapour 

 would ultimately escape by filtration through the pores of the 

 rock, rapidly consolidating on exposure, and splitting up by the for- 

 mation of shrinkage cracks into such rude prismatic blocks or cakeB 

 as are seen to cbaracterize the surface of lava-streams of this coarse 

 grain. 



One word upon the varying mineral and chemical characters of 

 Lavas. I cannot but think that far too much importance has been 

 attached to these distinctions, especially by the German geologists. 

 By many of these, as in the instance of Baron von Eichtofen, whose 

 classification of Volcanic Eocks was lately reviewed by me (Geol. 

 Mag., Vol. vi., p. 518), these differences, in their minutest peculi- 

 arities, have been laid down as determining the relative age of the 

 respective rocks. There can be no greater source of error. It is 

 certain that many varieties of trachyte and basalt, and rocks of in- 

 termediate mineral character — that is to say, with a greater or less 

 proportion of acid or basic elements in their composition — are often 

 found succeeding each other as products of the same volcano, in no 

 definite series ; sometimes one class, sometimes another, having been 

 first ejected. Nay, they are to be seen occasionally, though rarely, 

 to pass into each other in the same mass, just as some granites are 

 found locally passing into syenite, and this again into greenstone. 

 There are even lavas, as for example that called Peperino, so much, 

 employed in buildings at Naples, in which zones or lenticular 

 blotches of different mineral character alternate throughout the rock, 

 the augitic matter having apparently separated itself from the more 

 feldspathic by a process of segregation during the efl&ux of the lava. 

 And there need be little doubt that what has taken place in this 

 instance on a small scale has frequently occurred on the large one, 

 within the focus of a volcano, during the, perhaps repeated, pro- 

 cesses of alternate fusion and re-crystallization, to which a mass of 

 subterranean lava has been probably exposed, under varying circum- 

 stances of temperature and pressure. (See Volcanos, pp. 129-132.) 



If these views are correct — and it is for younger field-geologists 

 than myself to prove or disprove their truth by a close examination 

 of volcanic districts — they cannot but throw much light on the 

 nature and character of the heated material that underlies the crust 

 of the globe, and makes itself known, both in outward flows of 

 lava, and in the penetration of the fractured crust, by intrusive 

 veins, dykes, and protruded bosses of crystalline rock — in the 

 deeper-seated syenites, granites, porphyries, and serpentines, no less 

 than the trachytes, greystones, and basalts of sub-aerial eruptions. I 

 hope, therefore, to be excused for repeating here ideas on this subject, 



