Basic Eruptive Rochs. — Merrill. .^1 
groundmass and in crystals of but one generation, the augites in 
two generations, the feldspars rarely in two, and the olivines in 
but one. The intratellural origin of these last is manifested Ijy 
their presence in large numbers in blebs of considerable size in 
small offshoots from the main dikes, which are sometimes of not 
more than an inch in width and otherwise completely aphanitic. 
The olivines are not, however, equally conspicuous in all the dikes. 
In the first two met with going southward from the mill (the first 
about thirty and the second fifteen inches wide) olivine is scarcely 
evident to the unaided eye. In the third it is in certain pai'ts of 
the dike verj' aljundant; in the fourth scarcely evident again, and 
in the fifth and sixth extremely abundant and in large- bleljs, as 
mentioned above. That, however, the rocks are essentially the 
same and must be portions of the same magma, is evident from 
their similarity in mineral, as well as chemical composition. 
There are certain differences in the slides from different dikes 
that are worthy of note. The larger dikes, as would be expected, 
show as a rule a more granular structure, though this is not a 
universal rule. In the second dike below the mill (28525)* the 
idiomorphic nature of the hornblendes and augites of the ground- 
mass is less distinctl}' marked than in the first. A sample from 
this dike yielded Prof . Packard 40.26% SiO.^. The third dike 
(28526) is a typical camptonite, and excepting the numerous large 
olivines, is indistinguishal)le in the section from the original dia- 
base of dike No. 1, at Campton, New Hampshire, as described by 
Hawes. t The fourth dike, three feet wide, (28527) is quite simi- 
lar, but the slides show in places a structure bordering upon 
ophitic. The fifth, 2 feet wide (28528), shows also a tendency 
toward an ophitic structure and there is a much less distinct sep- 
aration of the minerals into individuals of two generations. The 
olivine is here the only mineral porph3'riticalIy developed. The 
sixth dike, 2^ feet wide (28529 and 39194), is the mo.st striking 
of all, and the one to which owing to its richness in olivines ni}' 
attention was first attracted. J 
In the hand specimen this is a dense dark gray aphanitic rock 
*The numbers given are those of the specimen as entered in the 
Museum catalogue. 
tAm. Jour. Sci., 3d, xvii, 1879, p. 147. 
JThis is the rock referred to by Prof. Kemp in his paper "On the 
Dikes Near Kennebunkport, Maine." Am. Geologist, March, 1890, p. 
138. 
