PELROGRAPHICAL PROVINCE OF HSSEX COUNTY _A77 
they are remnants of an anticlinal fold of the Cambrian sedi- 
ments, perhaps produced by the intrusion of the eruptive granite 
magma from beneath them.’’* 
A further fact, which is in favor of laccolithic differentiation, 
is the radical difference of opinion of the two observers, Wads- 
worth and Sears, who are best acquainted with the region, as to 
the order of succession of the igneous rocks. Leaving the 
dikes out of consideration, and adopting the nomenclature of 
this paper, Wadsworth? gives it as follows, beginning with the 
earliest: gabbro and diorite, quartz-syenite, nepheline-syenite, 
granite, and rhyolite. Sears,3 on the other hand, gives granite, 
quartz-diorite, essexite, diorite, nepheline-syenite, and quartz-sye- 
nite, gabbro, rhyolite. This argument is not conclusive, since the 
differences of opinion are perhaps explicable on the ground of 
lack of good exposures, etc., but in the case of two good 
observers, who made a careful study of the region, they are sug- 
gestive of the fact that there is no ‘‘order of succession” in the 
usual sense of the term, and which we would not expect to find 
in the case of a differentiating laccolithic mass of magma. 
Lastly, it may be mentioned that the rocks are just what we 
might expect to find as the differentiates of a magma, such as 
this was, when laccolithically differentiated. The transition types 
which exist between the various members, and the “schlieren,” 
such as those in the diorite of Marblehead, are also in favor of 
this hypothesis, as is also the greater abundance of dikes in the 
granites as compared with the diorites, since it seems reasonable 
to suppose that many cracks formed from below would penetrate 
the granite, and not the overlying diorite zone. 
Several objections may readily be brought against this hypoth- 
esis, among which may be mentioned, the asymmetric character 
of the complex and the absence of quartz-syenite between the 
granite and diorite, where we should expect to find it. The first 
tA similar structure is apparently suggested by Hopss (Amer. Geol., Vol. 
XXII, p. 110, 1899) for the area about Waltham, southwest of our region. 
? WADSWORTH: Geol. Mag., p. 209, 1885. 
3 SEARS: Bull. Essex Inst., Vol. XXVII, p. 109, 1895. 
