THE COBALT SERIES; ITS CHARACTER AND ORIGIN 127 
The argillite is much finer grained than the greywacke, consisting 
of exceedingly minute fragments of quartz and feldspar imbedded 
in a chloritic cement. Small quantities of sericite, epidote, and 
carbonate are also commonly present in all of these rocks. 
Arkose and quartzite.—The greywacke and argillite are usually 
replaced on passing upward by arkose or quartzite, the transition 
taking place by a gradual increase in the feldspar and quartz 
content or by an alteration of beds of the two rocks. The arkose 
Fic. 2.—Photomicrograph of conglomerate matrix. Crossed nicols. 20 
and quartzite are firmly cemented sands which, when examined 
under the microscope, are found to consist of round, angular, or 
subangular fragments of quartz, or of quartz and feldspar along 
with small quantities of calcite, sericite, epidote, pyrite, and other 
minerals. They are generally stratified, may show ripple marks, 
are locally cross-bedded, and in places contain well-rounded pebbles 
of quartz and jasper in lenticular aggregations. 
Upper conglomerate—Wherever the Cobalt series has a con- 
siderable vertical thickness, the arkose and quartzite are overlain 
