152 THE GEOLOGY OF MINNESOTA. 



[Tuff. 



Mc(j. Evidently a clastic rock of rounded and sub-rounded grains, in a thin or 

 shaly stratification, some of the individual grains being lighter and some darker than 

 the general tone of the specimen, which is a light brown, varying to a grayish, with 

 a tint of green. Some of the larger pieces, which are also the darker ones, are 

 apparently from some aphanitic rock. Ready effervescence follows the application 

 of a small drop of hydrochloric acid. 



Mic. The rounded grains are of various composition, but the major part of 

 them consist largely of quartz, perhaps mingled sometimes with a little mica and a 

 little plagioclase. This quartz may be distinguished in two categories: 



1. Clear limpid quartz, the grains apt to be somewhat angular, supposed to be 

 a primary portion of the grit accumulated to form the rock; not common. 



2. Quartz derived from an alteration of volcanic glass. This appears in three 

 conditions: 



(a) As clear quartz with a single orientation, constituting a border that sur- 

 rounds a more or less confused and cloudy nucleus; some of the larger grains are of 

 this character. 



(b) As quartz more or less clouded and flecked by minute opaque particles, such 

 quartz occupying the whole area of a grain, and with one or with several orientations. 



(c) As microlitic or globular quartz, the minute globules not having crystal 

 outlines nor clearly separate orientation, but crowded and grouped in the central 

 parts of gi'ains which (as in a) have clear quartz in their peripheries, or in some 

 other places. 



Calcite forms some of the rounded grains, some of it showing the peculiar, 

 marked cleavage characteristic of that mineral. It is also sometimes as an important 

 part of the devitified grains. 



Other grains are made up now of a greenish substance, probably some form 

 of chlorite, which occasionally has a finely fibrous structure perpendicular to the 

 circumference of the grain. This, however, is generally destitute of such a structure. 

 It is but slightly less abundant than the silicified grains. 



Rarely a grain is simply gray, and semi-opaque, and its nature cannot be 

 determined. 



Magnetite and pyrite (?) are not common, though a ferruginous dust which 

 appears to be of hematite or of magnetite, is scattered generally through, not only 

 some of the individual silicified grains, but more abundantly in the finer matrix. 



The most striking feature of the slide, as seen under the microscope, with a low 

 power, is the non-resolvable nature of most of the grains, though sufficiently 

 translucent to show light. 



Two sections. 



