and Volcanic Substances. 231 
Analysis of common Hornblende. — Augit (Frascati.) 
Silex - - 42, 48 
Alumine - - 12 5 
Lime - . 11 24 
Magnesia - - 2 25 8 75 
Oxyd of Iron - 30 12 
Manganese = 25 1 
Water - - 75 
Trace of potash - 
The black basalts wherein either hornblende or augit 
predominates, do not decompose by the action of air and 
moisture so easily as the greyish trachytic varieties which 
contain more felspar. The olivine or peridot of the cellu- 
lar basalt, also decomposes easily into an ochreous pow- 
der. By long exposure, all kinds of basalt gradually de- 
compose into a yellowish fertile soil. Terras or Puzzuolana 
is generally regarded as decomposed Lava, or decomposed 
basalt, containing much iron: and argillaceous iron ores 
have been found of equal value for like purposes. 
Basalt Lavas, and Basalt floetz trap, moreover agree in 
other particulars, noticed by M. Conybeare, 3 Geol. trans. 
208. The newest formations are the lowest—the least ele- 
vated among the strata: they occupy vallies, plains, the 
foot of mountains : but Lavas, and floetz-trap porphyries 
and basalts are found covering high mountains as in Saxony 
and other parts of Germany, and in South-America. 
- Among the decided volcanic formations, the least an- 
cient, are the least elevated ; but the flcetz trap surmounts 
all others. Of all other formations, the degree of consoli- 
dation decreases, as they are of later origin: thus the most 
crystallme are the primitive: next come the transition 
which are sub-crystalline ; then the compact, coarse, and 
earthy : while in the fleetz trap, even where it rests on 
chalk, the crystalline character of the older rocks often ap- 
pears. 
Whin-Dykes which belong to the fleetz trap basalt, and 
amygdaloid, differ from all other mineral veins, by travers- 
ing all rocks indifferently as Lava does; while other mine- 
ral veins are associated with particular rocks. 
