PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 759 



Gbbro.] 



Meg. Rather coarse grained, gray. . 



Mic. Except that the augite has been converted to hornblende, this is like the 

 Duluth gabbro, the large crystals of labradorite also being somewhat altered by the 

 introduction of zoisite and sericite. There seems never to have been any olivine in 

 the rock, but there are several crystals of apatite of the first consolidation. The 

 hornblende lies about the feldspars in an ophitic manner. One section. 



Age. Probably Archean. 



Remark. This rock has the characters of the Cabotian gabbro when affected by 

 contact with clastic acid rock. N. H. w. 



No. 1685. GABBRO. ( Hornblendic. ) 



"A darker, more magne tiled condition of No. 1684." 

 Ref. Annual Report, xxi, page 155. 



Meg. A very dark, greenish-gray, almost black, coarse-grained rock, composed 

 of feldspar, much hornblende, and perhaps some augite. Apparently a uralitized 

 gabbro. No section. 



Age. Probably Archeau. u. s. G. 



No. 1685A. GABBRO. 



The same place as No. 1684. 



Ref. Annual Report, xxi, page 155. 



Meg. Similar to No. 1684. 



Mic. The augite is not entirely changed to hornblende in this slide, and there 

 is a remarkably frequent supply of idiomorphic apatites. These lie in the hornblende, 

 in the magnetite and in the unchanged augite. One section. 



Age. Probably Archean. 



Remark. This rock is allied petrographically to No. 1678, the gabbro quarried 

 at Little Falls. N. H. w. 



No. 1686. GABBRO. (fiaitsni<ritizcd.) 



Same place as No. 1684. Somewhat quarried. 

 Ref. Annual Report, xxi, page 155. 



Meg. Nearly white. 



Mic. The rock consists, in places, entirely of zoisite, but in general the 

 feldspars also contain numerous scales of sericite. Through the whole sometimes the 

 remnants of feldspar crystals can be seen to be present by the continuance of the 

 interrupted straight belts of albite twinning. This indicates that this alteration is 

 not due to shearing, since if sheared the twinning lines would be likely to be distorted. 

 One section. 



Age. Probably Archean. N. H. w. 



