DR BOUE' ON THE GEOGNOSY OF GEllMANY. 107 
I shall now conclude this long letter, by enumerating the 
characters which appear to me most distinctive of the two 
kinds of igneous rocks, those formed under, and those 
formed above the water. 
1. The igneous rocks formed under water, at least those 
posterior to the chalk formation, do not rise into hills of so 
great a height as those formed above the surface ; and, in 
general, the first class of rocks must have certainly, in all 
periods, had more difficulty in attaining the same height 
as the second. 
2. The first class produce veins or dikes more easily, and 
in greater number, than the second. 
5. When the first class of rokcs form a kind of coulee 
or stream, these streams seem generally not to unite the 
length and the small breadth of the streams (coulees) above 
the water. 
4. The rocks of the first class are generally more com- 
pact than those of the second. 
5 The basalts of the first class are often intimately unit- 
ed with basaltic tuffs, and the porphyries with some kinds of 
felspathic breccia ; an appearance which is almost entirely 
unknown in the basalts produced above the surface of the 
water, because in them the small pieces which form the tuffs 
had been ejected by the volcanoes under the form of rapiUi. 
6. Rocks with the vitreous character abound much more 
in the igneous rocks formed above water, than in those 
formed under it. 
7. The igneous rocks formed under water, contain many 
substances, produced by infiltration, unknown in the other 
class of rocks, and more frequently also substances pro- 
duced by sublimation. 
8. The basalts formed under water show imbedded, very 
often, pieces of the neighlpouring rock, which are more or 
