PEPROGRAPHICGAL PROVINCE OFVESSEX COUNTY. 479 
case the rocks are all genetically connected, in the other pos- 
sibly not. 
Conclusion and summary.— My view, then, of the structure of 
the complex and genetic relationships of the rocks of Essex 
county is as follows. The igneous area represents possibly a 
section through an elongated, irregular, anticlinal, laccolithic 
mass, with a N. NN. E.—S.S.W. trend, intruded into already dis- 
turbed Cambrian strata. The plutonic igneous rocks represent 
the products of a laccolithic differentiation in this magma, and 
not successive intrusions. The magma was rather acid, rich in 
alkalies and ferrous iron, and possibly did not differ materially 
from the calculated composition given on page 472. This course 
of differentiation produced primarily and most abundantly the 
granites, quartz-syenites, and diorites, with their corresponding 
aschistic dikes as Machschube. A secondary local differentiation 
of the more basic differentiates of this process gave rise to the 
foyaites and essexites, while the rocks of the paisanite-tinguaite 
series are probably diaschistic dike forms of these. At a much 
later period, after erosion had removed much of the covering of 
the granolites, the rhyolites were erupted, from some unknown 
vents. Lastly, here, as so often elsewhere, the diabases were 
injected into the much cracked complex. 
Comparison with other regions—So much space has been 
devoted to the previous discussion that a few words must suffice 
for this topic. From the references to the Christiania region 
throughout the paper and comparison of the analyses, the general 
correspondence between the two is evident. There are differ-_ 
ences in details, such as the more acid and somewhat less soda- 
rich character of the Essex county rocks, abundance of diorites 
here and of laurvikites and laurdalites in Norway, etc. But it may 
be said that the general course of differentiation was essentially 
much the same in both, a fact which supports the theory that 
the variations of rocks are largely subject to physico-chemical 
laws, and are not fortuitous and due to the composition of the 
surrounding rocks and other external circumstances, as Johnston- 
Lavis and Becker would have us believe. 
