32 CATALOGUE OF PHILOSOPHICAL APPARATUS, &c. 



Drawer 42. Domite, a variety of Trachyte, from Puy de Dome, &c., near 

 Clermont. 



N. B. Trachyte is essentially a felspathic rock, consisting however of those varieties of the 

 felspathic family, which contain the largest amount of Silicic acid, in proportion to that of 

 the bases with which it is combined, so that it approaches more nearly to granite than ordi- 

 nary lavas do. 



From Abich's Geol. Beob. it appears, that the rock is made up principally of glassy Felspar, 

 and of Albite, both which minerals possess the same composition, so far as regards the 

 proportion of Silica to Base, though differing in the nature of the Base present ; in both of 

 them i atom of Potassa, or some equivalent base, being combined with i atom of Silica ; and 

 i atom of Alumina, or of some equivalent base, being united with 3 of Silica ; its composition 

 admitting of being represented by the following formula : 



RO, Si O 3 + R 2 O 3 , 3 Si O 3 . 



With these minerals variable proportions of quartz are usually intermixed, as is the case 

 with granite, but generally in smaller quantities than in the latter. See the first chapter of 

 my Descriptions of Volcanos. 



Drawer 43. Volcanic rocks associated with the Tertiary formation of the 

 plain of Limagne, near Clermont. 



Drawers 44, 45. Antient volcanic rocks, chiefly Trachytic, including deposits 

 from the Thermal waters of St. Nectaire, and of Mont Dor. 



N. B. I have distinguished these lavas into antient and modern, according as they are inter- 

 sected by the valleys of the country, or follow their slope; the former being antecedent, the 

 latter subsequent, to the formation of the valleys in which they occur. 



Drawer 46. Antient volcanic rocks, near Clermont. 



Drawers 47, 48. Antient volcanic rocks, Cantal. 



Drawer 49. Modern volcanic rocks, including compact as well as scoriaceous 

 varieties, Puy en Velay. 



Drawer 50. Modern volcanic rocks from the Vivarais. 



Clinkstone and other ancient ones, Mount Mezen. 



For a description of these rocks see a memoir of mine published in 

 the Edinburgh Philosophical Journal for 1820 1821, and like- 

 wise my Description of active and extinct Volcanos p. 22. 



N. B. The latter work is referred to in subsequent parts of the Catalogue under the title of 

 " Volcanos." On the geology of central France Mr. Scrope's memoir may also be consulted 

 with advantage. The panoramic views which accompany that volume are particularly instruc 

 tive. 





