23 
Felstone, Claystone, and Pitchstone are respectively compact, earthy or 
glassy Felspar. Those rocks which enclose large crystals in their general 
mass are termed Porphyritic. 
The Hornblende group comprises the following — Syenite, a variety of 
Granite, in which Hornblende replaces Mica ; and when the Quartz is 
absent the rock becomes Greenstone. Hypersthenite and Diallage are 
chiefly Silicates of Iron and Magnesia. Serpentine is a Hydrous Silicate of 
Magnesia. Diorite, a mixture of Albite and Hornblende, is abundant in 
Corsica. Melaphyre is chiefly a Silicate of Alumina. Diabase and 
Aphanite are varieties of Greenstone. 
The Volcanic division has similar groups of Felspathic and Hornblendic 
rocks. Trachyte is the prevalent rock of the Felspathic group. It is 
composed of Silicate of Alumina with potash and soda. It forms the mass 
of the Drachenfels. Pearlstone is Trachyte with small globular concre- 
tions. Compact soda-felspars are called Clinkstone or Phonolite. Obsidian 
is glassy Trachyte and Pumice has the same composition but with a 
cellular structure. The Hornblende group contains Dolerite, a granular 
mixture of Labradorite and Augite. Basalt is very compact Dolerite. A 
variety less compact than Basalt is termed Anamesite. 
The Metamorphic rocks were then described. Gneiss and Mica-schist 
were originally sediments of clay and sand, derived probably from Granite. 
They are of great variety, arising from the prevalence of either quartz or 
mica or hornblende in the original rock. Clay-slate, when uniform in 
structure admits of being cleaved into very thin slates. This quality was 
shown to arise from pressure exerted in a direction perpendicular to the 
planes of cleavage. 
The annexed table is reduced from the larger one which was used to 
illustrate the paper and is appended for reference. 
