C. H. Smyth, Jr. — Metamorphism of a Gabhro. 275 



corresponds to the lateral. The plates are parallel with each 

 other, are from one to six inches apart, and occur over areas of 

 several square yards. Wherever this structure is developed, 

 the gneiss has a very peculiar surface. The rock immediately 

 adjacent to the hornblende has been rendered particularly lia- 

 ble to weathering, so that the plates all occupy shallow grooves, 

 separated by corresponding ridges. As a result, the surface of 

 the rock presents an appearance which may be described as 

 similar to that of corrugated roofing. The distance between 

 the grooves corresponds to that between the hornblende plates, 

 their depth reaches an inch or more. As a rule, the plates of 

 hornblende, on account of their resistant character, project 

 slightly above the bottom of the groove. 



The grooves, following the hornblende plates, often curve 

 considerably, and when the curves become sharp they com- 

 monly pass into small shear zones. In other cases, instead of 

 finding relief along distinct zones, the pressure has crushed 

 masses of the rock into a rather fine breccia. 



It is evident that this development of hornblende plates is, 

 like the foliation, a secondary phenomenon. For, not only 

 would it be very difficult to explain as a case of primary band- 

 ing, but the plates occur only in the gneissoid portion of the 

 rock, showing that their formation has some connection with 

 the gneissoid character. While so much is clear, the precise 

 relation between the platey structure and the foliation is not 

 so evident. The hornblende plates occur in a distinctly 

 gneissoid rock, and make a high angle with the foliation, but 

 they are not themselves affected by this foliation, rather cut- 

 ting across and interrupting it. The intimate connection 

 between foliation and platey structure, however, indicates 

 that they should be regarded as phases of one period of meta- 

 morphism, rather than as resulting from two distinct periods 

 with different conditions. 



It may be worth while, in passing, to note the resemblance 

 presented by this phase of the gahbro to a metamorphosed sedi- 

 mentary rock. In the absence of the massive and transitional 

 portions, the platey structure, with attendant grooving of 

 weathered surfaces, might well be regarded as representing orig- 

 inal bedding, crossed at high angles by secondary foliation. 

 There is afforded here an instance of the apocryphal nature of 

 parallel structures in crystalline rocks. 



Petrographic Description. 



In specimens of the normal gabbro the feldspar is pale gray 

 or purplish, altering to green. The ferro-magnesian constitu- 

 ents are dark green to black, often green in the center with 

 black margins. 



