hayes and egkel.] IRON ORES OF CARTERSVILLE DISTRICT, GA. 235 
sive coarse-grained rock, containing large porphyritic crystals of feld- 
spar (microcline), in a groundmass of plagioclase feldspar, muscovite 
mica, and bine quartz. Some portions of the rock have undergone 
considerable alteration, by which it has been converted into an augen- 
gneiss. This area of Corbin granite at one time probably formed an 
island, since it is surrounded, in part at least, by rocks derived from 
its waste. These are feldspathic conglomerates in which the blue 
quartz and the porphyritic crystals of microcline, which characterize 
the granite, can be readily distinguished. In some places the transi- 
tion from granite to conglomerate is so gradual that it is difficult to 
determine the exact boundary between the two formations. The 
development of the gneissoid structure in the granite evidently took 
place after it was deeply buried by sediment, for the alteration of the 
latter is even more marked than that of the granite itself. Wher- 
ever the granite is not bordered by coarse conglomerate or quartzite 
it is in contact with black graphitic slates, which generally overlie the 
coarser sediments. 
These conglomerates and slates associated with the granite belong 
to the Ocoee series, which reaches its greatest development in eastern 
Tennessee and western North Carolina. No fossils have yet been 
found in the rocks of this series, although man} 7 of them are only 
slightly altered. They contain limestones and slates similar to por- 
tions of the adjacent valley formations, but the latter are always found 
to contain more or less abundant traces of life. In the absence of 
fossil evidence their age can not be definitely determined, but on 
structural evidence, obtained chiefly in Tennessee, they are believed 
to be Lower Cambrian with possibly some pre-Cambrian. 
The rocks of the Ocoee series generally show an increasing degree 
of metamorphism toward the southeast; and within a few miles of 
this region they pass into schists and gneisses, the original form of 
which, whether igneous or sedimentary, can not be readily deter- 
mined. This increased metamorphism toward the southeast is due in 
part to the greater compression which that region has suffered, and 
in part to the presence of considerable bodies of various igneous rocks 
which have been intruded into the sedimentary beds. These intrusive 
rocks present considerable variety in composition, varying from 
extremely basic diabase to acid granites. The most common variety 
is a diorite, which was among the earlier intrusions, and has been 
snbsequently converted for the most part into amphibolite-schist. 
Two belts of this basic schist pass across t lie southeastern corner of 
the district. Its southeastern corner is occupied by the Acworth 
gneiss, which, like the Corbin granite, is probably Archean in age, 
and formed the foundation on which the oldest sediments of the region 
were deposited. 
Structure. — In common with other portions of the southern Appa- 
lachian region, the Cartersville district has been subjected to intense 
