692 Dikes of the Hudson River Highlands. 
The reconnaissance survey of Dr. Britton! and Mr. Merrill over 
this line, in connection with the New Jersey survey in 1885, brought 
to light some, at first sight, rather obscure dike rocks which were 
entrusted to the writer for determination. Their interesting char- 
acter, however, encouraged further investigation in the field by him 
the past summer, and this has led to the following results. In the 
October School of Mines Quarterly (Vol. IX., p. 33), Dr. Britton 
has outlined the results of his work. In brief he subdivides the 
Archean into a basal “ Massive Group,” a middle “ Iron-Bearing 
Group,” and an upper “Gneissic and Schistose Group.” These 
members, as remarked by Dr. Britton, shade more or less into 
one another, nor is it always easy to sharply define the individuals. 
In the particular section under consideration we have especially to 
deal with the iron-bearing and massive members as containing the 
dikes, for the area which is notable for their absence is regarded by 
Dr. Britton as belonging to the upper Gneissie and Schistose 
series. 
Of the dikes between Tomkins’ Cove and Jones’ Point the writer 
is prevented by the unfortunate loss of a note-book from speaking 
with the same accuracy of location as in regard to those remaining. 
There are some present, though in but few instances. One slide 
shows a hornblende-porphyrite very similar to one found inland 
and about two miles west, to be described and figured in a pape? 
forthcoming in the American Journal of Science for September. This 
is probably an outlying dike of the Cortlandt series. 
From Jones’ Point to Iona Island the railway skirts the Dunder- 
berg. The rocks are gneisses, with evident laminations that strike 
on the average N. 40 E. Just above Jones’ Point, and at intervals 
for two miles to the north, they are seamed with dikes. Eleven 
such masses were noted. They vary from six inches to twenty feet 
wide, and are in almost all instances very badly altered on the 
exposure. In fact they weather much worse than the enclosing 
walls, and frequently show a recess from which the specimen has 
to be fairly dugout. Under the microscope enough of the structure 
remains to show that they were in all cases either diorites or horn- 
blende-porphyrites, consisting either of a holocrystalline aggregate 
of hornblende, plagioclase and magnetite, or of a ground mass n° 
1 See Rept. State Geol. New Jersey, 1886, p. 74. 
