PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 255 



Diabase. Aporhyolyte.] 



Remark. There is now no doubt, judging from an examination of numerous 

 instances of contacting red rock and the basic eruptiyes, that the peculiar characters 

 of this rock are the result of small masses of the red-rock series being included in 

 No. 200 at the time of the intrusion, and after complete or partial fusion the dis- 

 semination of the elements into the basic rock. 



The rock has evidently suffered a great change since its intrusion, giving rise to 

 the zeolites, the "chalcedony" and the copper, and cai'rying forward the alteration 

 of the feldspars, but a considerable portion of the alteration was probably accom- 

 plished before the cooling of the mass. The three sections examined by Wadsworth 

 all contain particles of metallic copper. Two others made since show the same. 

 The amount is not sufficient, here, to warrant any economic exploitation. It only 

 shows that the circulating waters at the time of its deposition had access to con- 

 siderable copper-bearing rock masses, and it is not unlikely that such sources may 

 yet be found in the vicinity. The original source is more likely to be in connec- 

 tion with the reddish-brown felsyte which this basic eruptive cuts than in the erup- 

 tive itself or in some associated detrital rocks. 



In further examination of these radiating spheruliths two sections were made by 

 M. Marchand of a thickness as low as 0.015 millimeters. In these the augite shows, 

 between crossed nicols, a light yellow tint, the feldspars are neutral, and the spher- 

 uliths, when cut properly, have a yellow color. Some part of the thomsonite shows 

 no color, or only a gray tint, but darkening when parallel with the threads; and 

 some beautiful, perfect, radiated spheruliths, having a negative elongation, have, 

 intermediate between the centre and the periphery, a sharply defined concentric 

 band of fibres parallel with those on either side of the band which are of positive 

 elongation. On the introduction of the quartz plate of the sensitive tint the quad- 

 rants are contrastedly colored, and those in this narrow band are contrasted in an 

 opposite direction from those outside and inside of the band. 



Some calcite also has formed in association with this thomsonite. N. H. w. 



No. 201. APORHYOLYTE? 



Prom the mine at Fall river. It is cut by No. 200. 



Ref. Annual Report, ix, page 54; Annual Report, x, page 141; Proceedings American Association for the 

 Advancement of Science, vol. xxx, page 164. 



Meg. A very fine-grained, compact, hard, siliceous rock containing small, red- 

 dish feldspar phenocrysts. Quartz phenocrysts are very rare, only two or three being 

 shown on the hand specimen. No section. 



Age. Cabotian; red-rock series. 



Remarks. This rock seems to be of the same general character as the rock of 

 the Great Palisades i. e., an acid surface lava and it is also thought to be of the 

 same age. u. s. G. 



