APPEARANCE OF ROCK IN THE FIELD. 259 
in a previous paper,* but, being of fundamental importance in the pres- 
ent inquiry, demands more extended consideration. 
In the exposure on the east bank the rock is dark gray or nearly black, 
sometimes having a greenish tinge. The only mineral conspicuous to 
the unaided eye is a lustrous, dark-brown mica, in scales varying from 
minute up to half an inch or more indiameter. In certain sheared por- 
tions the rock is much altered, though retaining its deep color, with a 
rather more marked green tinge. Elsewhere it appears to be quite fresh, 
though surprisingly soft when hammered. This softness results from a 
change of some kind in the rock, which is further indicated by the pres- 
ence of numerous veinlets of calcite in the form of “satin spar.” 
On the west bank the dike presents a totally different aspect. Instead 
of being nearly black in color, it is light yellowish brown, near the shade 
sometimes called golden brown. At the same time it is so thoroughly 
disintegrated that it may be easily scooped out with the hands. Indeed, 
so far as field evidence is concerned, there is nothing to indicate that the 
rock was once like that on the opposite bank except its occurrence as a 
clearly defined dike cutting the Calciferous‘‘ sand-rock ” and the presence 
in it of occasional lumps of the parent rock and of abundant scales of 
mica, like those in the other rock, though considerably decomposed. But 
these very facts, which to a casual observer might appear of little moment, 
are quite sufficient, aside from the confirmatory evidence furnished by 
microscopic examination, to establish the essential identity of the two 
rocks. 
The weathered dike, with a width of from 26 to 30 inches, is exposed 
vertically about 15 feet in the wall of the creek gorge (see plate 18), and 
to the bottom of the exposure the degree of weathering is quite uniform. 
There can hardly be a doubt that it extends to a greater depth ; but while 
this, as well as the nature of the rock as it recedes from the vertical face, 
could probably be determined by a moderate amount of excavation, it 
would involve the destruction of a beautiful example of weathering in a 
region where such examples are rare. As it now stands, the exposure is 
terminated below by a heap of the weathered rock resting upon a shelf 
of Caleiferous, partly produced by blasting. The refuse blocks and other 
detritus completely hide the downward continuation of the dike. The 
upper 15 feet of the ‘‘sand-rock ” show the normal weathered surface of 
a calcareous rock, while the lower 10 feet have the fresh surface resulting 
from blasting. 
The weathered dike has receded from the face of the cliff a distance of 
from two to five feet, and the fissure walls left exposed are weathered to 
* Am, Jour. Sci. (4), vol. ii, pp. 290-292. 
