56 On the Minerals of Trap and the allied Rocks. 
the fusibility of the zeolites and their easy destruction by heat and 
by volcanic gases, sulphureous and muriatic, we should a@ priort 
say that they could not be formed under such circumstances. 
6. Besides, as we have stated, none of the proper constituents of 
trap or basalt—or the minerals disseminated through these rocks, 
—contain water. They are all anhydrous. The minerals form- 
ed accidentally in furnaces are anhydrous. The constituents of 
granite, syenite and porphyry are all anhydrous. It is only those 
minerals which are found in geodes or seams that contain water. 
Of equal importance is the fact, that none of the essential con- 
stituents of these rocks have ever been found in these geodes or 
cavities along with the zeolites, as might have been the case had 
they been formed together, by segregation or otherwise. Neither 
feldspar, although so abundant, nor augite, nor chrysolite, have 
been found filling, like zeolites, or with them, the cavities of 
amygdaloid. There is then a wide distinction between the an- 
hydrous constituents of these rocks, and the hydrous zeolitic 
minerals. 
A few zeolites have been found in granite or gneiss, but they 
are so disseminated that they can be shown to be of more modern 
origin than the rock, and to have resulted from some decomposi- 
tions of true granitic minerals. They differ entirely in their 
mode of distribution from the feldspar, garnet, &c. of granite. 
Along with a decomposing feldspar, it is not unusual to find 
stilbite in the cavities formed by the decomposition. 
Zeolites also have been found disseminated through the tex- 
ture of basalt, clinkstone, &c., like the feldspar, augite, &c. But 
the proportion varies widely, and in some parts of the same bed 
they are found to be wanting; so that we have sufficient reason 
for classing these disseminated zeolites with those in the cavities, 
as formed or introduced by infiltration. 
7. Bearing upon this subject, it should be observed, that the 
constituents of amygdaloidal minerals are, in general, those of 
the containing rock. Silica, potash, soda, alumina, are found in 
the feldspars; lime, magnesia and iron in augite or hornblende; 
iron and magnesia in chrysolite. These are all the constituents 
needed, except a little baryta for one species. The feldspar decom- 
poses readily and gives up its ingredients, its potash or soda, silica 
and alumina; the same is true of augite and chrysolite, which af- 
ford magnesia, lime, silica and iron. With water to infiltrate, we 
