THE SLATES. 57 



series would be classed by Dana as mica slates, but by Von Cotta and Zirkel 

 as argillaceous mica shists. Rocks of this character are described by Mr. 

 Caswell from Rapid Creek under numbers 64, 65, 67, and 71. 



The argillaceous slates (argillaceous mica schists, Caswell) are among 

 the most prominent of the slaty rocks, and in color they are usually dark 

 brown or gray. By reason of their compact and uniform structure they 

 are less easily decomposed by atmospheric influences than the micaceous 

 schists of the western group. They are so fine in texture that it is difficult 

 to detect with the eye the elements of their composition. They appear 

 generally to contain fine particles or grains of mica, quartz, and argillace- 

 ous mineral, and they always give an argillaceous odor when moistened 

 with the breath. Their most argillaceous varieties split readily into thin 

 sheets, resembling in texture and appearance roofing or school slates, or 

 become soft and are readily cut like slate pencils. In many places, as on 

 upper and lower Spring Creek and lower Rapid Creek, the clay slates are 

 exceedingly dense and fine, and owing to peculiar cleavage planes break 

 up into slabs or splints, which may be seen in many places piled up like 

 cord-wood. These pieces are cut with difficulty by a knife and give a 

 strong metallic ring when struck. 



Light gray or drab, fine, soft, talcose, argillaceous slates, containing 

 minute particles of mica, have been found in some localities, as on middle 

 Rapid and Box Elder Creeks. 



From the distinctive clay slate there is frequently a passage into a 

 very silicious slate, close-grained and banded with thin laminae of different 

 colors ; and this silicious slate sometimes passes into true quartzite, hard, 

 uniform, and compact. Silicious slates associated with quartzites are well 

 exhibited in the canons of Box Elder, Rapid, and Spring Creeks. 



In several localities the silicious slates contain interlaminated with 

 them immense quantities of iron, almost always specular oxide. On Box 

 Elder Creek, a ridge some 400 feet in height is composed of a vast deposit 

 of silicious hematite, which was estimated to be from 800 to 1,000 feet in 

 thickness across the upturned strata. Occasional bands of almost pure 

 specular hematite several inches in thickness are found in the mass with 

 frequent layers of highly crystallized micaceous hematite. The body of 



