ACID SCHISTS IN THE MONA SCHIST AREA. 159 



which is no doubt a new product, derived from tlie plagioclase of nn older 

 rock. Epidote in small, almost colorless grains is ([uite conunon in and 

 between the new plagioclase particles, sometimes as single individuals, 

 sometimes as clusters of grains that are so thickly crowded as to be 

 almost opaque. Plates of the common yellow-green epidote are occasion- 

 ally met with, and crystals of zoisite are common in the altered plagioclases 

 of some sections. As a rule, ilmenite and leucoxene are not so widely 

 spread in these rocks as they are in some of the other schists. In the 

 schists derived from dike material and from the compact and coarse-grained 

 lavas leucoxene is abundantly present, whereas in the banded schists, 

 supposed to be altered tuflfs, and in the amphibole-schists, it is uncommon. 

 In a rock from 1100 steps^ N., 100 steps' W., SE. corner of sec. 2, T. 48 N., 

 R. 26 W., however, the section is sprinkled with little black particles of 

 ilmenite, each one of which is surrounded by a rim of colorless leucoxene. 

 These schists are like those described by Williams" from the "Brook 

 section" west of Marquette. Thus far they have been found only in the 

 southern halves of T. 4S N., R. 2h W., and T. 48 N., R. 26 W., although 

 they no doubt exist in other portions of the northern greenstone area. It 

 is impossible at present to decide whether these schists are squeezed tuffs 

 or squeezed lavas," but they are no doubt mashed rocks derived from basic 

 volcanic material of some kind. 



ACID SCHISTS. 



In a number of i)laces within the area of the Mona schists the green 

 schists are associated with light-colored rocks that are very like certain 

 schistose acid dikes that cut across the greenstones. There is great diffi- 

 culty in determining whether these light schists were derived from eruptive 

 porphyries or from their tuffs. In many instances the latter is sujjposed to 

 be the case. The larger decomposed fragments that lie in the tine-grained 

 groundmass of these rocks are so badly shattered, and the different pieces 

 near together fit into one another so imperfectly, that it W(iuld seem hardly 



' In this volume locations will freqiieutly be given from the southeast corners of sections, in steps 

 at the rate of 2,000 per mile. 



'^The ■'■reenstone-schist areas of the Menominee and Marquette regions of Michigan, by G. H. 

 Williams : Bull. U. S. Geol. Survey No. 62, 1890, pp. 156-157. 



