56 GEOLOGY OF THE BLACK HILLS 



fiful expanses of treeless and well-grassed parks that are found in the south- 

 western and western part of the schist area. The parks occupy irregular and 

 discontinuous tracts along the western margin of the Archaean region, and 

 extend from Box Elder Creek to the headwaters of French, Red Canon, and 

 Amphibious Creeks. One of the smaller of them with a granite ridge in 

 the distance is shown by the view in Golden Valley, Figure 4. 



Where the mica of the schists becomes less prominent and quartz takes 

 its place, or where strata of quartzite or beds of granite predominate among 

 the schists, the surface of the country becomes more rugged and peaks or 

 ridges of harder rocks stand out above the surface as rough and soilless 

 reefs. In the area where granite is the prevailing rock an excessive rough- 

 ness and ruggedness is given to the surface, which then becomes a mere 

 succession of sharp ridges and towering peaks. 



Characteristic samples of the schistose rocks will be found described in 

 Mr. Caswell's report under the numbers 1, 13, 24, 27. 



The eastern or slate series occupies probably a larger portion of the 

 Archaean area than do the schists of the western series. Its predominant 

 rocks are distinguished by their fine and uniform texture, and though they 

 nowhere have the true slaty cleavage making an angle with the bedding, 

 they are finely laminated and commonly split on their stratification into 

 tolerably thin pieces. They are quite distinct in their structure from the 

 coarsely crystalline schists of the western series, and are fairly distinguished 

 as slates, although the term does not apply in its more restricted sense. 

 They do not exhibit the variety of character observed in the rocks of the 

 western group, but consist mainly of fine silicious slate, clay slate (argilla- 

 ceous mica slate), argillite, talcose slate and quartzite. With these are inter- 

 polated many seams of quartz and specular iron ore. Characteristic speci- 

 mens of the slate have been submitted to Mr. Caswell, and though their 

 structure and composition are not readily discernible by an inspection with 

 the naked eye, under the microscope they are in general found to have a 

 true crystalline constitution and to be distinguished from the rocks of the 

 western series rather by the fineness and minuteness of their structure than 

 by a difference of composition. A large portion of the rocks of the eastern 



