PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 791 



Diabase. Gabhro.J 



No. 1811. DIABASE. 



From the vertical cliff forming the west shore of Sickle (Chicago) bay, north shore of lake Superior; S. 

 W. J4 sec. 20, T. 62-4 E. 



Ref. Annual Report, xxii, page 7. 



Meg. A black, rather coarse-grained diabase (or gabbro), resembling the Beaver 

 Bay diabase. No section. 



Age. Cabotian (Beaver Bay diabase?) u. s. G. 



No. 1813. GABBRO. 



West side of Double bay, at the point; N. W. % sec. 15, T. 62-4 E. 

 Ref. Annual Report, xxii, page 7. 



Meg. A coarse-grained, dark-gray gabbro, possibly containing olivine, with large 

 crystals of augite. No section. 



Age. Cabotian (Beaver Bay diabase?) u. s. G. 



No. 1814. GABBRO. (Ophitic.) 



Prom the hill range at Double bay, at the west end of the near hills. 

 Ref. Annual Report, xxii, page 7. 



Meg. Rather coarse grained and gabbro-like. 



Mic. Owing to considerable alteration, due, apparently, to long weathering, this 

 rock presents some interesting features. While a part of the augite, in large plates, 

 showing an ophitic relation to the feldspar, is perfectly preserved, other grains have 

 been changed entirely, and, along with an accompanying alteration in the olivine, 

 there have resulted considerable prehnite, epidote and antigorite. The feldspars have 

 also lost their integrity and some interstitial parallel growths have formed, some 

 being apparently new feldspars, and others an undetermined zeolite(?). At the 

 same time the titanic iron present has taken the sagenite structure of rutile, and 

 within the mesh, as well as elsewhere throughout the slide, appears leucoxene. The 

 prehnite encloses the epidote ophitically, the latter being in form of small idiomorphic 

 crystals. One section. 



Age. Cabotian. N. H. w. 



No. 1814A. GABBRO. 



" White-weathering lumps and patches in No. 1814." 

 Ref. Annual Report, xxii, page 7. 



Meg. Rock in general similar to No. 1814. The feldspars are white, perhaps 

 saussuritized. No section. 



Age. Cabotian. u. s. G. 



Remark. Nos. 1814 and 1814A illustrate the manner of segregation of the feld- 

 spars from the rest of the rock into irregular masses and groups, causing a remark- 

 able petrologic variation, indicating the possible origin of all the " anorthosyte " 

 masses. N. H. w. 



