44 BULLETIN OP THE UNIVERSITY OF WISCONSIN 



the exposure of volcanic breccia at Mr. Hewitt's place in 

 the northeast i of the northwest i of Sec. 21. 



Specimens from some of these blocks have a large num- 

 ber of small pheuocrysts while others show but few. 

 These phenocrysts are a light colored feldspar and usually 

 plagioclase. It is possible that this black quartz kerato- 

 phyre has its origin in the porphyry areas to the northeast, 

 but it may be that a closer examination of the area would re- 

 veal it in place. 



Sericite Schist.— Wherever the quartz keratophyre comes 

 in contact with the quartzite, there occurs a zone of sericite 

 schist from 150 to 200 feet wide. These schists are a dy- 

 namic alteration of the quartz keratophyre and they grad- 

 ually pass into the latter, and hence are not, as Irving ' 

 supposed, closely related to the magnesian schists which 

 occur at Devil's Lake. The schists vary in color from the 

 reddish brown of the quartz keratophyre to a grayish white. 

 In texture they are quite friable and are easily cleaved. 

 They carry a considerable number of feldspar crystals, 

 some of which have weathered out leaving cavities that 

 are usually lined with iron oxide. In some specimens iron 

 pyrite is seen. Some of the grayish white schists contain 

 many greenish brown areas of cherty-like material, which 

 the microscopic examination shows to be secondary quartz, 

 the rock being a pseudo-breccia. 



Besides the schists at the contact, there occurs within 

 the area of quartz keratophyre in the north half of the 

 northw^est i of Sec. 21, a large area of schist (Fig.l, Sec. 

 IV), which is capped by sandstone and conglomerate, and 

 which, like the schist at the contact, has a dip of 75° or 

 80° to the north. In all respects it is like the schist at 

 the contact, except that feldspar crystals in the hand speci- 

 men are rare. 



Volcanic Breccia.— There occur within the eruptive area 

 several good examples of volcanic breccia. Within the 



iGeol. of Wis.. Vol. II: page 513. 



