336 H. N. EATON 
The pre-Cambrian-Triassic boundary from north of Laurel 
Ridge to Cocalico is approximately the southern base of the 
mountain. ‘The distinct contrast in color between the lighter sandy 
soil of the one and the red soil of the other affords a means of 
accurate mapping in cases where only the decomposition products 
are available. 
In the center of the hill east of Fritztown a small inlier of pre- 
Cambrian in Potsdam was found covering an oval area seven- 
eighths of a mile east and west by one-fourth of a mile north and 
south. 
THE PRE-CAMBRIAN ROCKS 
These comprise solid and basic gneisses cut by granite pegmatite 
in a complex similar to that occurring in the range at Reading and 
eastward. A study of thin sections shows the acid variety to be 
altered granite and the basic, mainly altered gabbro. From 
rather scanty field evidence the conclusion was reached that the 
granite gneiss is the younger of the two. Exposures showing the 
intrusion of one by the other are visible along the road and on both 
sides of the stream in the ravine directly southwest of the Insane 
Asylum. The direction of the foliation is N. 58° E.; dip 43° S.E. 
The close association of these two rocks is universal throughout the 
area, and no attempt has been made to differentiate them on the 
map. In general the gabbroic variety would appear to predominate 
in the east while granite gneiss is abundant in the western part. 
Diallage is the ferro-magnesium mineral in the gabbro gneiss. 
A little hornblende, presumably secondary, also occurs. Magnetite 
is quite abundant. 
Diorite gneiss covers a small area in the center of the mountain 
near the headwaters of Furnace Creek. Its relations to the other 
rocks are unknown, but it is believed to be of limited occurrence. 
The granite gneiss is characterized by an abundance of alkali 
feldspar, more or less primary hornblende, an absence of biotite 
and a scarcity of muscovite. 
Foliation is not marked in hand specimens of the gneisses, and 
in many slides can barely be detected with low magnification. The 
grain is uniformly medium. 
These gneisses doubtless correspond very closely in composition 
