Lavis — On the Structure of Bocks. 145 



and more perfect. This rock may be traced by gradations to a 

 syenite-like rock, in which the amorphous magma is entirely con- 

 verted into formed matter. In the basic lavas, which are identical 

 in composition with the above pumices, the sanidine only occurs as 

 very small crystals or microliths, as the magma rising quickly to 

 the surface has little time to partially crystallize under pressure. 

 On the contrary, after extrusion, the lava will cool very much 

 slower than the pumice, so that the prolonged remit will be highly 

 favourable to development of orthoclase microliths, and even small 

 crystals. These facts are well borne out by Vesuvius in its pumices 

 and modern lavas, whilst the outflows of Phase IV., following 

 immediately on a pumiceous phase, hold an intermediate place 

 with regard to their monoclinic felspars. It is not an uncommon 

 thing in basic pumices to find sanidine 1 crystals eroded, enclosed in 

 others, which in turn may exhibit eroded surfaces, and again be 

 enclosed in a third crystalline shell with well-defined facets. The 

 orientation of each crystal being different from that which it coats, 

 or is covered by. It is evident, therefore, that this mineral must 

 have undergone a series of vicissitudes which must have taken 

 a far longer time than was occupied in the eruption and cooling of 

 this product of an explosive eruption, and must have required 

 more quietness than could occur in the expansion and ejection of 

 pumice. This latter example I take to be an important argument 

 in favour of the hydro-thermal, or plutonic, formation of ortho- 

 clastic felspar in a magma cooling under great pressure. Another 

 fact also of deep interest is the very extensive replacement of 

 sanidine in the Yesuvian pumices of Phases III. and IV., by 

 leucite in those of Phase VII. and the lavas, as these are the two 

 principal competitors for the potash. If the granite and syenites 

 of the Yal di Fassa, and the latter of Skye, described by Scrope 

 and Geikie respectively, are really subaerial expansions, which 

 I doubt, we must suppose them to have been nearly completely 

 crystallized before eruption. Porphyries, no doubt, are erupted 

 granites, which had undergone much crystallization before their 

 extrusion. Even in the most vitreous rocks, such as the obsidian 

 and obsidian pumice of Lipari, where the latter, although, as a 



1 H. J. J. L. "Geology of Mt. Somma and Vesuvius," &c. Q. J.G. S., Jan., 1884, 

 p. 71. 



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