328 Geological Society : — - 



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 IVIount, near 

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

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

 In the Yesuvian 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 di- 

 stinct 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 approached one another solidification proceeded more rapidly, 

 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, with scat- 

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

 rally some porphyritically imbedded felspar crystals or fragments of 

 such, both orthoclase and plagioclase. In polarized light, on cross- 

 ing the Nicols, the base breaks up into an irregular-coloured breccia, 

 the colours changing to their complemcntaries on rotating cither of 

 the prisms. 3. Pinely bedded ash, when highly altered, is in some 

 cases undistinguishable in microscopic structure from undoubted 

 felstone. 4. Ash of a coarser nature, when highly altered, is also 

 very frequently not to be distinguished from felstone, though now 

 and then the outlines of some of the fragments will reveal its true 

 nature. 5. The fragments which make up the coarser ash-rocks seem 

 generally to consist of felstone, containing both orthoclase and pla- 



