GEOLOGY OF THE SCHROOX LAKE QUADRANGLE 55 



Table 4 Thin sections of gabbro and diabase 





No. 39, i mile southeast of Minerva; no. 40, i^ miles east of Bailey 

 pond and near the mapped inclusion of granite; no. 41, middle of Texas 

 ridge gabbro area; no. 42, i l / 2 miles northeast of Lester dam; no. 43, 

 top of SaywO'cd hill; no. 44, one-third of a mile north of summit of Say- 

 wood hill. 



In the above table, nos. 41 and 42 are more typical of the gabbro 

 of the quadrangle, while nos. 39 and 40 are rather more special 

 or acidic facies. The more normal rock is therefore an olivine- 

 bearing hypersthene gabbro or norite. The garnet, which is no 

 doubt of secondary origin, mostly forms granulated borders about 

 granulated hypersthene. In slide no. 41 the biotite mostly forms 

 reaction rims about magnetite, and granulated garnet forms rims 

 around granulated hypersthene in nos. 41 and 42. Nearly colorless 

 diallage in no. 41 exhibits wonderful parting and schillerization 

 inclusions. The labradorite of nos. 41 and 42 are filled with tiny 

 dustlike dark inclusions. 



Special descriptions. The Texas ridge mass is the finest large 

 scale example of a gabbro stock in the quadrangle. It covers an 

 area of approximately 3^ square miles. An almost continuous 

 outcrop occurs along the whole crest of the ridge. Near the south- 

 ern end of the ridge a considerable mass of well- foliated pinkish 

 gray granite forms an inclusion in the gabbro (see map). Imme- 

 diately south of this inclusion the gabbro is notably variable, being 

 mostly moderately foliated, but some big ledges are medium grained 

 and very massive. None of this rock exhibits a diabasic texture. 

 No. 42 of the table shows the mineral contents of a thin section 

 of the more common rock, and this is seen to be a distinctly acidic 

 or dioritic facies even carrying considerable orthoclase. The near- 



