THE MONA SCHISTS. 157 



which arc known to have been derived l)y the aUeration of basic eruiitive 



rocks." ' 



Normally these schists show in thin section large or small slieaf-like 

 bundles of bluish-green hornblende scattered through the slide indiscrim- 

 inately or aggregated into groups with irregular outlines and frayed edges, 

 and embedded in a groundmass consisting of nuxch decomposed plagio- 

 clase and a mosaic of colorless grains of albite and quartz. With the 

 hornblende, chlorite is frequently associated, the areas occupied by the 

 two minerals sometimes having the outlines of an amphibole or a pyroxene 

 crystal. Much leucoxene is observed in most sections, and not infrequently 

 granular ei)idote is intermingled with the components of the mosaic. In 

 addition to these substances, which are well defined, there are certain 

 obscurely outlined i)lagioclases, which i)resent between crossed nicols the 

 shapes of sharp-edged fragments. In none of the slides of these rocks has 

 anything been detected that may be i-egarded as a waterworn sand grain. 



The plagioclase, whether in the fragments or in the indefinite areas 

 that serve as a sort of groundmass to nnu'h of the hornblende, is altered 

 to epidote, sericite, chlorite, and calcite. The mosaic filling the interstices 

 between everything else is in all probability secondary, as it not infre- 

 quently fills little veins cutting through the hornblende, chlorite, and altered 

 plagioclase. This mosaic is like that described by Lossen" in the schistose 

 diabases of the eastern Harz, which have been shown to owe their foliation 

 to mashing. 



The Marquette banded schists show the structure sometimes of massive 

 rocks and sometimes of pyroclastic ones, but more frequently they exhibit 

 no structure from which their origin can lie inferred. Their composition, 

 however, is that of diabases. Their field aspects are verj- diff"erent from 

 those of most schistose diabases. The rocks are banded, like sedimentary 

 ones. A possible explanation of these opposite sets of characters is that 

 the rocks exhibiting them are water-deposited elastics of volcanic origin, 



'The greenstone-schist areas of the Menominee and Marquette regions of Michigan, by 

 G. H. Williams : Bull. U. S. Oeol. Survey No. 62, 1890, p. 154. 



"-Zeitschr. Deutsch. geol. Gesell., Vol. XXIV, 1872, p. 730; Jahrbuch K. preuss. geol. Landesan- 

 atalt, 1883, p. 640; 1884, p. 528. 



