210 THE GEOLOGY OP MINNESOTA. 



[Aporliy.h te, 



in the sections, but only two feldspars are seen. These are very small, and their 

 character cannot be determined. A microchemical preparation, with hydrofluosilicic 

 acid, of one of the feldspar phenocrysts showed large amounts of both soda and 

 potash, with a very small amount of lime. The feldspar is probably tniortJiochise. 



Two sections. 



Age. Cabotian. 



Remark. This rock and those numbered 139 and 140 are from the Great 

 Palisades. These rocks are all regarded as acid lavas which were once glassy; in 

 fact, some of the specimens of No. 140 still contain glass, and some show distinct 

 perlitic cracks. Some of these features are described under No. 140. Compare also 

 Nos. 812 and 813. u. s. G. 



No. 139. APORHYOLYTE. 



So taken as to express the character of the rock of the bulk of the Palisades; of the same character as 

 No. 138. 



Ref. Annual Report, ix, page 35; Annual Report, xiii, pages 100 (No. 158), 103; Bulletin viii, page xxxiii. 

 (See No. 533.) 



Meg. A reddish brown, aphanitic rock, containing small phenocrysts of quartz 

 and feldspar, the latter pinkish to white in color. The rock is filled with irregular 

 cracks and under the hammer breaks up into small irregular nodules. It is difficult 

 to get a fresh fracture of large dimensions. 



Mic. The groundmass of the rock is similar to that of Nos. 68 and 138. The 

 particles of iron ore and the small cloudy feldspathic particles occur as in these rocks, 

 but the poikilitic quartz of the groundmass is of much smaller size. The sections 

 show distinct flowage structure, the lines of flow being brought out by streaks, 

 which are more or less filled with the iron ore. The sections show only a few of the 

 phenocrysts. The quartzes are small, angular or rounded, and one individual shows 

 an embayment filled with the groundmass. The feldspars are clouded and altered. 

 A microchemical preparation, with hydrofluosilicic acid, of these feldspar phenocrysts 

 showed large amounts of soda and lime, and but little potash. The feldspar is 

 thought to be oligoclase. From the analysis of the whole rock, given below, the 

 amount of lime and soda, especially of the former, is seen to be very small in com- 

 parison with the potash, and for this reason we might expect a more acid feldspar, 

 orthoclase or anorthoclase. It would seem, however, that the feldspars tested were 

 some of those first formed and thus contained a large percentage of lime. 

 Two sections. 



