C. H. Smyth, Jr. — Metamorphism of a Gabbro. 279 



The relations of hornblende and scapolite to the history of 

 the rock present some points for further consideration, As 

 has been stated, both occur abundantly in the most massive 

 portion of the rock, while the hornblende seems still more 

 abundant in the somewhat crushed portions, decreasing again 

 in the gneiss. The scapolite remains about the same in quan- 

 tity in the massive and crushed portions, but disappears almost 

 entirely in the gneiss, having been found in only one specimen. 

 In the case of the hornblende it is possible, though not proba- 

 ble, that the relations stated are only apparent, resulting acci- 

 dentally from the particular set of specimens examined. But 

 as to the scapolite there seems to be no doubt. These phe- 

 nomena lead to the conclusion that the formation of the scaly 

 hornblende and the scapolite took place under conditions dif- 

 ferent from those required for the production of the gneiss. 

 The latter is a clear case of dynamic metamorphism, with 

 development of a pronounced parallel structure, and complete 

 recrystallization, the previously formed scaly hornblende crys- 

 tallizing into larger masses of the same mineral and, partly, 

 into pyroxene as well ; while the scapolite is changed back 

 into feldspar. In other words, in the case of these minerals 

 intense dynamic metamorphism has brought about a direct 

 reversal of the earlier process which led to their formation. It 

 is evident that the different varieties of the rock represent 

 different stages of metamorphism, and that the work of the 

 earliest stage is, to a large extent, undone by the latest stage. 



The first stage is marked by the formation of scapolite and 

 some scaly hornblende, with little or no sign of crushing, the 

 probable agents of change being pressure, heat and solutions. 

 The latter factor is of importance in connection with the pos- 

 sible addition of chlorine in the conversion of feldspar into 

 scapolite. The familiar theory of Judd* to account for this 

 alteration certainly cannot be applied to the present case, what- 

 ever value it may have elsewhere. In the gefleckter gabbro, 

 or dipyre diorite of Norway other writers have pointed out the 

 connection between the alteration of feldspar to scapolite and 

 the apatite veins. The probable derivation of chlorine from 

 the vein-filling solutions has been pointed out, and has been 

 recently accentuated by Yogtf in a lucid manner. In the case 

 under consideration there is an absence of any distinct con- 

 duit for chlorine-bearing solutions, and yet it seems possible 

 that chlorine from a deep source may have been carried into 

 the rock by the gradual passage of water under heavy pressure, 



* J. W. Judd : On the processes by which a plagioclase feldspar is converted 

 into scapolite, Min. Mag., viii, 1891. 



*• f J. H. L. Vogt : Beitrage zur genetischen Classification der durch magma- 

 tische Differentiationsprocesse und der durch Pneumatolyse entstandenen Erzvor- 

 kommen, Zeits. f. prakt. Geol., 1895, p. 456. 



