474 HENRY S. WASHINGTON 
successive injections of liquid magmas and the differentiates* of a 
more deeply seated magma, only part of which was released from 
the reservoir, or are they the differentiates zm set of a body of 
magma which was injected in a more or less homogeneous con- 
dition from below? Is the differentiation, in other words, 
‘deep magmatic,” or “‘laccolithic ?’? Very thorough and care- 
ful field study is necessary to decide this question, study 
which it has not been possible for me to undertake. At the 
same time, certain considerations seem to point to the conclusion 
that the differentiation at Essex county was laccolithic, and that 
the complex may possibly be regarded as a laccolith. I can 
only point out very briefly the facts on which this conclusion 
rests, leaving the further study and settlement of the question to 
others. 
Comparatively very few differentiated laccolithic masses have 
been studied, but those which we know best show a basic border 
and more acid interior, with, in some cases, an intermediate zone 
of medium composition. Prominent examples of these are 
Brandberget, in Gran;3 Carrock Fell+ in England, and the 
especially beautiful and instructive ones described by Weed and 
Pirsson, notably, Square Butte,’ Yogo Peak,® and Bear Paw 
Peak? in Montana. 
On looking at the geological map of Essex county,® it is 
seen that the main granite area is to the east, extending in 
large patches from Cape Ann westward,and ending in this 
*I use this term as synonymous with and more convenient than “‘ differentiation 
product.” It is formed analogously to the word soéz‘e. 
? BROGGER: Quart. Jour. Geol. Soc., Vol. L, p. 29 ff., 1894, and Erupt. gest. d. 
Christ. geb., Vol I, p. 153, 1894. 
3 BROGGER: Quart. Jour. Geol. Soc., Vol. L, p. 31, 1894. 
4H{ARKER: Quart. Jour. Geol. Soc., Vol. L, p. 311, 1894, and Vol. LI, p. 125, 
1895. 
5 WEED and Prrsson: Bull. Geol. Soc. Am., Vol. VI, p. 389, 1895. 
© WEED and Pirsson: Am. Jour. Sci. Vol. L, p. 467, 1895. 
7 WEED and Pirsson: Amer. Jour. Sci., Vol. I, p. 351, 1896. 
8 The map here given is copied from the revised one of Mr. Sears,some omissions 
being made on account of the greatly reduced scale. An attempt has been made to 
show approximately the small foyaite area along the Beverly Shore. 
