ARCHEAN GRANITES. 267 



Here and tliere in the granite may be found masses of dark gray to 

 green rock. Some of tliem have been in planes along which movement has 

 taken place, and are extremely altered, and have become schistose. Others 

 have been much altered, but no actual motion appears to have occurred in 

 them, so that they are still massive. These rocks are generally basic — 

 some are even ultrabasic — and vary from basalts to peridotites. They 

 are intrusive in the granite, but how much younger than the granite they 

 may be is not known. The schistose basic rocks presumably belong to 

 a period of eruption later than the Lower Huronian, and correspond to 

 the dikes described in Chapter IV. The freshest basalts are presumably 

 of Keweenawan age and are similar to those described in Chapter VI, 

 under the heading "Keweenawan." 



Microscopic characters. — Under the microscope the granite is found to 

 be either a mica- (biotite-) granite or a hornblende-granite. This last is the 

 predominant rock. It varies by loss of quartz to a syenite. Grrant" has 

 described still a different facies of the granite of Saganaga Lake — a fluorite- 

 granite which he observed upon an island in Saganaga Lake. The essential 

 minerals are mica, hornblende, quartz, orthoclase, and plagioclase. With 

 these occur as accessor}^ minerals, and in very small quantity, some apatite, 

 sphene, and magnetite. These possess their usual characters and show the 

 relations common to such minerals in the granites. All of the rocks are 

 considerably altered. The usual secondary minerals — calcite, sericite, 

 actinolite, epidote, and chlorite — liaA^e been produced and are present in the 

 sections examined. The granite varies somewhat in textural character from 

 the normal granite to a granite-porphyry. 



In the granite-porphyries the phenocrysts are quartz, hornblende, and 

 plagioclase. Around some of the feldspar phenocrysts there is occasionally 

 micropegmatitic intergrowth of the feldspar with quartz. These minerals 

 are distinctly of the first generation, and lie in a moderately fine-grained 

 groundmass of hornblende, feldspar, and quartz of the second generation, 

 in striking contrast in size to those of the first generation. 



«Geol. and Nat. Hist. Survey of Minnesota, Twentieth Ann. Kept., 1893, p. 89; Geol. and Nat. 

 Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, p. 323. 



