FORMATION OF VEINS. 711 



Herschel brought forward the argument that since there is an increase of tem- 

 perature for every sixty feet of descent in the earth's crust, if strata should ac- 

 cumulate over a region in the sea to a depth of 10,000 feet, the heat would rise 

 accordingly into the stratified mass; and, as the same temperature would exist 

 at a depth of sixty feet as before, there would be accordingly in the lower part of 

 the mass the same elevated temperature that existed 10,000 feet below the former 

 surface, — this being a means of raising heat from below without disturbance, and 

 a degree of heat that in some circumstances might be sufficient for metamor- 

 phism. But if metamorphism had actually taken place in this way we should 

 expect to find sections showing horizontal or slightly-disturbed metamorphic beds, 

 and a gradual transition through a series of such beds to an absence of meta- 

 morphism ; but this has nowhere been observed. The great Appalachian faults 

 (p. 707) are direct testimony against the theory. It is remarkable that even in 

 the case of the Azoic rocks, formed in a period in which it is supposed the crust of 

 the earth was thin, there are no examples of metamorphic horizontal beds (p. 144) ; 

 they lie folded or tilted beneath horizontal Silurian strata in Canada (p. 134). 



4. Metamorphism of metamorphic rocks. 



Metamorphic rocks are not proof against further metamorphism. 



Among the Azoic rocks of northern New York (in Fowler, De 

 Kalb, Edwards, Russel, Grouverneur, Canton, and Hermon, St. 

 Lawrence co.) there are extensive beds of a kind of soapstone 

 (called Rensselaerite) which has in places the cleavage of pyroxene, 

 showing an alteration of pyroxenic and perhaps other rocks into 

 soapstone by some magnesian process ; and the serpentine of the 

 region may be of the same period of metamorphic change. Exam- 

 ples of the change of crystals and rocks to soapstone or serpentine 

 occur in the metamorphic regions of New Jersey and Pennsylvania; 

 and they are common in other countries. Again, in the Azoic of 

 northern New York, at Diana and other places in Lewis county, 

 there are beds of a soft compact rock which is sometimes worked 

 into inkstands, and resembles the agalmatolite of China; and at 

 one locality there are crystals of nepheline altered to this agalma- 

 tolite. These cases of the metamorphism of metamorphic Azoic 

 rocks may have taken place during the epoch of metamorphism 

 after the Palaeozoic era, when the rocks of New England were to so 

 large an extent crystallized. 



See further, on the history of this branch of science and its processes, a 

 Memoir by Daubree, translated from the French by T. Egleston, and published 

 in the Smithsonian Annual Report (8vo) for 1861. 



4. FORMATION OF VEINS. 



1. Veins. — Veins occupy either fissures intersecting strata, or 

 spaces opened between the layers of folded beds. They may result 



