108 Mineralogy and Geology of the White Mountains. 
Trap Dikes at the falls in Campton. 
Two miles north of Plymouth, at the falls of the Pemigewas- 
set, are some remarkable dikes. The whole bed of the stream is 
of solid rock, and the river which makes here a considerable fall, 
runs in several channels, separated by rugged rocks, each of which 
must be crossed by a dam to secure the whole power of the 
stream for manufacturing purposes, which is now in process of 
execution. 
The rock is granitoid, partaking in different mats of the varied 
character that belongs to the several members of this family, and 
perhaps may be called gneiss, as nearly as any specific name will 
apply ; course N. E. ; estimated dip 30° east. ‘This is, however, 
quite variable, as at dike No. 6, the dip of the strata is near 60°, 
and the cause is apparent. There is a group of dikes half a mile 
or more above these falls, which I had no opportunity of examin- 
ing, but hope the next season to visit them. My description at 
present is only of that group that occurs at the falls. The num- 
ber of dikes in this is seven, all of which may be examined very 
conveniently, unless it be No. 7, which, at certain heights of the 
water, is not quite as accessible as the others. The west bank 
is intersected by all these, which in some cases may be seen cut- 
ting the bed of the stream, the rocky island, and even the oppo- 
site bank; and all occur within a distance of a quarter of a mile.* 
No. 1. Just above the bridge; dark gray, and nearly black ; 
contains crystals of black hornblende and points of feldspar; erys- 
tals and films of iron pyrites in trap and associated rock, in small 
fissures extending from one to the other; some specimens fire 
with steel. Course E. and W., and cuts, as in Fig. 3, the island 
in the stream, and is seen in the eastern bank ; intersects veins 
of quartz. 
No. 2. (First below the bridge;) direction E. and W., and in 
upper part four feet wide ; divides a few feet from the water into 
two branches; the upper one is seen in the island ; structure very 
compact, like feldspar; fracture uneven; color light greenish gray ; 
powder almost a clear white; weathered surface dark brown; 
fires with steel ; translucent on the edges; effervesces abundantly 
(in powder) with dilute sulphuric, hydrochloric and nitric acids; 
* "The accompan accompanying sketch is not intended as a correct topographical view of 
the falls at Campton, but only as showing the manner in which the dikes occ 
The sketch was made wholly from memory. 
