Geological Society of London. 569 



G-BOLOGiOAL Society of London. — Novemloer 4, 1874. — John 

 Evans, Esq., F.R.S., President, in the Chair. — The following com- 

 munication was read : — 



"Notes on the Comparative Microscopic Rock-structure of some 

 Ancient and Modern Volcanic Eocks," By J. Clifton Ward, Esq., 

 E.G.S. 



The author stated at the outset that his object was to compare the 

 microscopic rock-structure of several groups of volcanic rocks, and 

 in so doing to gain light, if possible, upon the original structure of 

 some of the oldest members of that series. The first part of the 

 paper comprised an abstract of what had been previously done in 

 this subject. 



The second part gave details of the microscopic structure of some 

 few modern lavas, such as the Solfatara Trachyte, the Yesuvian 

 lava-flows of 1631 and 1794, and a lava of the Alban Mount, near 

 Eome. In the trachyte of the Solfatara, acicular crystals of felspar 

 show a well-marked flow around the larger and first-formed crystals. 

 In the Vesuvian and Albanian lavas leucite seems, in part at any 

 rate, to take the place of the felspar of other lavas ; and the 

 majority of the leucite crystals seem to be somewhat imperfectly 

 formed, as is the case with the small felspar prisms of the Solfatara 

 rock. The order of crystallization of the component minerals was 

 shown to be the following : — magnetite, felspar in large or small 

 distinct crystals, augite, felspathic or leucitic solvent. Some of the 

 first-formed crystals were broken and rendered imperfect before the 

 viscid state of igneous fusion ceased. Even in such modern lava- 

 flows as that of the Solfatara, considerable changes had taken place 

 by alteration and the replacement of one mineral by another, and 

 this very generally in successive layers corresponding to the crystal 

 outlines. The frequent circular arrangement of the glass- and stone- 

 cavities near the circumference of the minute leucite crystals in the 

 lava of 1631 was thought to point to the fact that after the other 

 minerals had separated from the leucitic solvent, the latter began to 

 crystallize at numerous adjacent points ; and as these points ap- 

 proached one another, solidification proceeded more raj)idly, and 

 these cavities were more generally imprisoned than at the earlier 

 stages of crystallization. In the example of the lava of 1794, where 

 the leucite crystals were further apart, this peculiar arrangement of 

 cavities was almost unknown. 



The third part of the paper dealt with the lavas and ashes of 

 North Wales ; and the author thought that the following points were 

 established : — 



1. Specimens of lava from the Arans, the Arenigs, and Snowdon 

 and its neighbourhood, all have the same microscopic structure. 

 2. This structure presents a hazy or milky-looking base, wdth scat- 

 tered particles of a light-green dichroic mineral (chlorite), and 

 generally some poi-phyritically imbedded felspar crystals or frag- 

 ments of such, both orthoclase and plagioclase. In polarized light, 



