758 



DYNAMICAL GEOLOGY. 



feldspathic gneiss, which usually has nearly straight lines of bedding 

 (or, of schistose structure) appears with these lines much flexed ; 

 then, in a complete zigzag, and on so small a scale that the hand can 

 cover the whole breadth of a flexure, — a condition indicating that 

 there had been a close approach to plasticity ; then, beyond, the lines 

 of bedding are all gone, and the rock is well characterized granite, 

 sometimes only for a few rods in extent, but sometimes for hundreds. 

 The best of architectural granite has had this origin. 



(3.) The heat for most metamorphic results was probably compara- 

 tively low, or between 500° F. and 1200° F. It was heat (4) in slow 

 and prolonged action, operating through a period that is long, even ac- 

 cording to geological measure. A low temperature, acting gradually, 

 during an indefinite age — such as Geology proves to have been re- 

 quired for many of the great changes in the earth's history — would 

 produce results that could not be otherwise brought about, even 

 through greater heat. 



The lower limit of temperature is sometimes placed much below 300° F. ; and for con- 

 solidation it may be rightly so. But there is definite evidence that it generally ex- 

 ceeded this. In the great faults of the Appalachians, 10,000 feet or more in extent, 

 Lower Silurian limestones are brought up to view, containing their fossils, and not 

 metamorphic; and in Nova Scotia the coal formation, though 15,000 feet thick, is not 

 metamorphic at base. Taking the increase of temperature in the earth's crust at 1° F. 

 for 60 feet of descent, 10,000 feet of depth would give 220° F. as the temperature of 

 the limestone before the faulting; and 1° F. per 60 feet of descent must be short of the 

 rate that obtained in the Carboniferous age. 



2. Moisture. — The fact that moisture was concerned is evident 

 from the inability of dry rocks to become heated without its presence. 

 Dry rocks conduct heat badly. Even a foot of fire-brick will confine 

 almost all the heat of a furnace, and a yard of solid lava as completely 

 the heat of the melted rock underneath. The presence of moisture 

 has further been demonstrated by microscopic investigations of the 

 metamorphic rocks themselves ; for as Sorby showed, water is com- 

 mon in minute cavities in the quartz, garnet, and some other minerals 

 of these rocks, and besides, as a result of the subsequent cooling, 

 vacuities occur with the water. Crystals of quartz containing water 

 in this way could not have crystallized from a state of fusion. 



The moisture engaged in producing the metamorphic changes was 

 for the most part that which existed in the sedimentary formation, 

 either (1) permeating the rock, or (2) confined between its layers; so 

 that the heat found the moisture which it needed for its diffusion and 

 for metamorphic work within the strata themselves. 



The average amount of moisture present in uncrystalline rocks, as 

 limestones, sandstone, shales, is over 3 per cent., exclusive of that be- 

 tween the layer. Take it at only 2*67 per cent., and the amount will 



