26 
PEOQRESS OF MICROSCOPICAL SCIENCE. 
Comparative Microscopic Bock-structure of some Ancient and Modern 
Volcanic BocIcs.—'Mt. J. Clifton Ward lately (Nov. 4, 1874,) read a 
valuable paper on the above subject before the Geological Society. 
He 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 Vesuvian 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 replace- 
ment 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 farther 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 crossing 
the Nicols, the base breaks up into an irregular-coloured breccia, the 
colours changing to their complementaries on rotating either of the 
prisms. 3. Finely-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 
