148 G. W. Hawes—FEruptive Rocks in New Hampshire. 
miles distant from the larger and more accessible town of Ply- 
The Pemigewassett river has here cut a gorge through 
a hill, and in the walls of this gorge the eruptive dikes are very 
conspicuous. The gorge is not long, and the dikes, five in 
number, are all embraced in a portion of it which is little more 
than a hundred yards in length. 
Attention was called to these dikes in 1837, by Professor 0. 
P. Hubbard of Dartmouth College.* His description of them 
is accompanied by a picture of the gorge, which shows their 
forms and relative position; but as the rocks could only be 
identified by microscopic examination, he did not attempt to 
elassify them. 
The rock through which the dikes intrude is mica schist, 
which presents its usual diversities, caused by variation in the 
proportion of the essential ingredients and the presence of acces- 
sories. The strike is northeast and the dip is variable. These 
rocks are considered to be as old as the Silurian, and Professor 
Hitchcock regards them as still older. 
ve dikes cut the schist almost at right angles; all are 
nearly vertical, and parallel to one another. A bridge has been 
built across the gorge from which all the dikes can be seen €X- 
cept the one directly under the bridge. From their position 
with reference to the schists, it is inferred that the fractures re 
sulted from the action of the same forces acting in the same 
way. Yet among these five dikes, there are found four very 
well distinguished rock species. I will describe these rocks 0 
the order in which they occur, beginning with the one highest 
up the stream. : 
ike No. 1, is seen only upon the left of the stream. Its 
about three feet wide; the rock is black in color, compact @ 
apparently nearly homogeneous. The study of some thin sec 
tions indicates that it is a diabase. It was originally a mixture 
of augite, a triclinic feldspar and titanic iron, but all its ingre- 
dients are partially altered. The augite is in proc 
alteration into hornblende; some of its grains being still intact, 
some being partially and others wholly altered. The feldspar 
the rocks described are placed together on a subsequent paz® 
Dike No. 2 is eight feet wide. The rock is black in color, 
* Am. Jour. Sei., I, vol. xxxiv, p. 105. 
