280 C. H. Smyth, Jr. — Metamorphism of a Gabbro. 



and probably at a high temperature. The whole question is r 

 however, an obscure one, on which the case under considera- 

 tion throws no light. At the same time that the scapolite was 

 forming, the conversion of pyroxene into hornblende began. 

 From such a process there would result, ultimately, a rock 

 composed of hornblende and scapolite, which would be mas- 

 sive, but there could not be formed a granulitic pyroxene- 

 hornblende gneiss. 



In the second stage, the effects of crushing are pronounced. 

 All of the constituents are granulated, and the rock becomes 

 more or less gneissoid. At the same time the scaly hornblende 

 increases in quantity, seeming to reach its maximum in this 

 phase of the rock. 



Finally, in the third stage, the rock undergoes complete 

 recrystallization, the new formed constituents being arranged 

 normal to the pressure that has crushed the rock, and thus pro- 

 ducing a pronounced gneissoid structure. In this final product 

 of metamorphism the recrystallization has removed all trace of 

 cataclastic structure. This is a point of some importance in 

 its bearing upon the question of the origin of gneisses, for it 

 shows the uncertainty of one of the criteria which have been 

 used in dealing with this problem. In his valuable paper on 

 the Laurentian of Canada, Dr. Adams'* has stated that the 

 gneisses with cataclastic structure are probably igneous, while 

 those lacking this structure have resulted from the crystalliza- 

 tion of sediments. While he gives abundant evidence to sub- 

 stantiate this view for the Canadian rocks, the case under con- 

 sideration shows that this criterion must be used with caution. 

 For here is an example of a gneiss derived from an igneous 

 rock, and yet without a trace of cataclastic structure. There 

 can be no doubt, moreover, that the converse is often true, as 

 there is no reason why the constituents of a rock formed by 

 crystallization of sedimentary material should not be crushed 

 by subsequent pressure, yielding cataclastic structure. 



That there have been three such stages of metamorphism, 

 with the results outlined above, seems to follow clearly from 

 the facts observed. As to the scapolite, the history is plain, 

 but in the case of the hornblende the writer accepts with some 

 hesitation the recrystallization into pyroxene, as this is a direct 

 reversal of the common alteration. Still, that such a change 

 has taken place is strongly indicated, and can be doubted only 

 by accepting a remarkable coincidence in the specimens studied. 



On comparing the normal gabbro with other rocks of the 

 family in the Adirondack region, it appears to be most closely 

 connected with a gabbro occurring in Pitcairn, St. Lawrence 



* F. D. Adams : A. Further Contribution to our Knowledge of the Laurentian, 

 this Journal, 1, p. 58. 



