504 ADDRESS OF T. STERRY HUNT. 
At the early periods in which the materials of the ancient crys- 
talline schists were accumulated, it cannot be doubted that the - 
chemical processes which generated silicates were much more ac- 
tive than in more recent times. The heat of the earth’s crust 
was probably then far greater than at present, while a high tem- 
perature prevailed at comparatively small depths, and thermal 
waters abounded. A denser atmosphere, charged with carbonic 
acid gas, must also have contributed to maintain, at the earth’s 
surface, a greater degree of heat, though one not incompatible 
with the existence of organic life. * These conditions must have 
favored many chemical processes, which, in later times, have 
nearly ceased to operate. Hence we find that subsequently to 
the eozoic times, silicated rocks of clearly marked chemical ori- 
gin are comparatively rare. In the mechanical sediments of later 
periods certain crystalline minerals may be developed by a process 
of molecular re-arrangement — diagenesis. These are, in the feld- 
spathic and aluminous sediments, orthoclase, muscovite, garnet, 
staurolite, cyanite and chiastolite, and in the more basic sedi- 
ments, hornblendic minerals. It is possible that these latter and 
similar silicates may sometimes be generated by reactions between 
silica on the one hand, and carbonates and oxyds, on the other, 
as already pointed out in some cases of local alteration. Such a 
case may apply to more or less hornblendic gneisses, for example, 
but no sediments, not of direct chemical origin, are pure enough 
to have given rise to the great beds of serpentine, pyroxene, stea- 
tite, labradorite, etc., which abound in the ancient crystalline 
schists. Thus, while the materials for producing, by diagenesis, 
the aluminous silicates just mentioned, are to be met with in the 
mud and clay-rocks of all ages, the chemically formed silicates; 
capable of crystallizing into pyroxene, tale, serpentine, etc., have 
only been formed under special conditions. 
The same reasoning which led me to maintain the theory of an 
original formation of the mineral silicates of the crystalline schists, 
induced me to question the received notion of the epigenic eri 
of gypsums and magnesian limestones or dolomites. The inte 
stratification of dolomites and pure limestones, and the aa 
of pebbles of the latter in a paste of crystalline dolomite, arè 
themselves sufficient to show that in these cases, at least, dolo- 
mites have not been formed by the alteration of pure limestone? 
* Amer, Jour. Sci II, xxxvi, 396 
