PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 549 



Diabase.] 



No. 795. DIABASE. (Porphyryte. ) 



"From the same, one-half way up. The same rock [as No. 794] with coarser feldspar crystals." 

 Ref, Annual Report, x, page 106. 



Meg. Similar to No. 796, but the phenocrysts are small and comparatively 

 scarce in No. 795. 



Mic. The slide is in general similar to Nos. 793 and 794, but contains one large 

 porphyritic feldspar. 



One section. 



Age. Cabotian. u. s. G. 



No. 796. DIABASE. (Porpliyryte. ) 



" From the top of the hill, 1,111 feet above lake Superior." See under No. 793. 

 Ref, Annual Report, x, page 106. Compare No. 456E. 



Meg. The rock is a rather fine-grained, gray diabase, similar to Nos. 793 and 

 794, but it contains numerous porphyritic plagioclases of a yellowish-gray color. 

 These phenocrysts are mostly from a half inch to an inch across. 



Mic. The groundmass of the rock is in general similar to Nos. 793 to 795, /. e., 

 it is composed of lath-shaped plagioclases, augite, olivine, iron ore and alteration 

 products. In addition there is a cloudy gray feldspar in irregular grains, which is 

 of later date than the lath-shaped feldspars, and which seems to have been the last 

 mineral to crystallize. All the feldspar, including that in phenocrysts, is more or 

 less kaolinized. 



Two sections. 



Age. Cabotian. u. s. G. 



No. 797. DIABASE. 



Taylor's Falls. 



Ref. Annual Report, x, page 106; Bulletin ii, page 114. 



/. A fine-graiiied, greenish-black diabase with marked areas of poikilitic 

 augite, which give a "lustre-mottled" character to the rock. One of the specimens 

 has a considerable area of " chalcledonic " quartz. 



Mic. The rock and sections are similar to No. 820. 



Three sections. 



Age. Cabotian. u. s. G. 



No. 79*. DIABASE. (Porpliyryte.) 



Taylor's Falls. 



Ref. Annual Report, x, page 106. 



A medium-grained, yellowish-green diabasic rock with porphyritic crys- 

 tals of brownish plagioclase. The rock has been considerably altered and has much 

 epidote developed in it. The specimen is crossed by a vein of epidote and "chalce- 



