﻿ANNIVERSARY ADDRESS OP THE PRESIDENT. 



crystals of the very fusible mineral augite. There is an. important 

 remark of Daubree, that, at high temperatures, so small a quantity 

 of water is sufficient to produce crystallization of silicates, that that 

 existing in clays, or even that mechanically contained in rocks, com- 

 monly termed the water of the quarry (eau de carriere), appears to 

 be all that is required to develope, when assisted by heat, very 

 energetic action. 



Metamorph ism . 



The term " metamorphic " was first given by Sir Charles Lyell in 

 1833, as a designation for certain of the older strata considered to 

 have been altered by subterranean heat. He states, in the last 

 edition of his 'Manual of Geology,' that by metamorphism he means 

 an action, existing in the interior of the earth at an unknown depth, 

 whether thermal, hydrothermal, electrical, or other, analogous to that 

 exerted near intruding masses of granite, which has, in the course of 

 vast and indefinite periods, and, when rising perhaps from a large 

 heated surface, reduced strata thousands of yards thick to a state 

 of semifusion, so that on cooling they have become crystalline like 

 gneiss. He enumerates as the principal metamorphic rocks — gneiss, 

 mica-schist, hornblende-schist, clay-slate, chlorite-slate, hypogene- 

 limestone, and certain kinds of quartzite. These rocks, he says, 

 when in their most characteristic and normal state, are wholly 

 devoid of organic remains, and contain no distinct fragments of 

 other rocks, whether rounded or angular, and that, however cry- 

 stalline they may become in certain regions, they never, like granite 

 or trap, send veins into contiguous formations, whether into an older 

 schist or granite, or into a set of newer fossiliferous strata. Here 

 then we have the term distinctly conjoined with a theory of the 

 agency by which the metamorphism is produced, and an equally di- 

 stinct restriction of the term to particular rocks. 



But such a restriction is now disregarded both in this country and 

 elsewhere, and the term metamorphic is applied to any sedimentary 

 rock, secondary or tertiary, which is altered from its original state 

 to a hardened or crystalline structure, without reference to any 

 theory of the agency by which the change was produced, however 

 diversified the nature of the rock may be. Thus, M. Coquand, in his 

 ' Traite des Roches,' published in 1857, has a whole family of Roches 

 metamorphiques, comprising thirteen species and no less than eighty- 

 eight varieties, as follows : — 



Mica-schiste, embracing. . 



. 11 varieties. 







Talc-schiste ,, 



• io 





2 



Clilorite-schiste „ 



5 





5 » 



Amphibolite-schiste ,, .. 



5 „ 





5 „ 



Argiloschiste „ 



9 





5 „ 



Calcaire ,, 



12 





..... 6 „ 



Dolomie „ 



6 „ 







Taken in its widest sense, the term might be applied to every altered 

 rock, be its age what it may ; for a structural change is as distinct 

 in many sandstones and limestones as in the older strata. Studer 



vol. xvii. d 



