30 
GEOLOGY. 
dition of rude crystallization. A second dike, of similar lithological character, with a trend IS. 
30° W., was found on the summit of an adjoining ridge. Neither of these intrusive dikes of 
trappean rock were over fifty feet in width. They apparently conformed in dip to that of the 
granite, which was not far from vertical. 
The relations of these rocks, and the peculiar form of the outcropping surface, will be under¬ 
stood by the inspection of the annexed sectional representation: 
SECTION NEAR WHITE CREEK. 
a, granite ; b b, trapdikes ; c c c, granitic, laminated, and probably metamorphic. 
From the appearance of the ridges for several miles to the eastward, and the character of the 
outlying rocks, 1 concluded them to he a continuation of this granite series. 
Before reaching “White creek,” an outcrop of dark-colored, highly crystalline rock, appa¬ 
rently intrusive, appears on the summit of an elevated hill but a short distance from the granite 
outcrops that have just been described. This rock, when seen from a distance, glittered in the 
sunlight as if it was filled with plates of mica. These reflections proceeded from flat cleavage 
surfaces of broad crystals of hornblende, one-half to three-quarters of an inch in length, forming 
the main constituent of the rock. The outcrop of this aggregation was so limited in extent that 
no observations upon the trend, or the associated rocks, could be satisfactorily made. 
On the top of the same hill, and west of the hornblendic rock, there are obscure outcrops of 
ferruginous quartz, intersected and crossed by a net-work of white quartz. Many loose frag¬ 
ments of red and jaspery quartz were picked up on the side of this hill. The whole locality was 
worthy of a more extended examination than it was in my power to make. 
White Creek to Ocoyo , or Pose Creek, 24| miles .—On the hanks of White creek, at the place of 
our camp, there are two or three nearly precipitous bluffs, one hundred to one hundred and fifty 
feet in height, with a steep slope of debris at their base. The color of these rocks, und the traces 
of stratification and absence of crystalline character, disposed me to regard them as altered sand¬ 
stone ; but a further examination before leaving camp led me to consider them as of igneous 
origin. They had Ihe appearance of stratified rocks set on edge. Stratified formations of sedi¬ 
mentary origin do, however, occur on the south side of the creek, and are first seen in the sides 
of low rounded hills bordering our trail. These beds were of argillaceous sandstone, and had a 
VALLEY BETWEEN TERTIARY HILLS. 
slight inclination towards the west. Our trail, for several miles, was among hills of this pecu¬ 
liar, rounded, smooth outline, characteristic of the sandstone formation. The subjacent granite 
