Dikes of the Hudson River Highlands. 693 
merely a structureless alteration product with occasional er; © 's of 
hornblende, plagioclase and magnetite. Pyrite is sometimes seen} 
the same is true of biotite and small prisms of apatite. The mag- 
netite shows indications of titanium. The Dunderberg exposures 
are rather conspicuously contorted and broken. The strike of the 
laminations of the gneiss, while generally northeast, cannot in all 
cases be determined. The dikes seem sometimes parallel with them, 
sometimes notably run across them. They may or may not have 
experienced some or all of the metamorphic processes through which 
the wall rocks have passed (an idea to be more fully developed 
later), but the porphyritie structure would indicate the contrary. 
Much in the way of contact influences, if anything, cannot be 
detected. They are not far from the neighboring Cortlandt series. 
They may have been connected with it. This at present cannot be 
affirmed or denied. 
North of the railway station at Iona Island, a belt of horn- 
blendie schist, with great masses of hornblende and epidote, is 
encountered, This association is quite typical of certain localities 
in the Iron Bearing group, and strongly resembles the same associa- 
tion of the two minerals to be seen at the Todd! Mine in Sprout 
Brook Valley, northeast from Peekskill. This last is on the line 
of strike from Iona Island. It is not surprising, as the writer 
was informed by a resident, that explorations have been made in 
the southwest, finding, however, nothing but lean ore, too poor to 
work. These outcrops possibly form a “range” similar to the 
well-known ranges of New Jersey, with the Croft and Stuart or 
Sunk mines at the extreme northeast. 
‘Between the Poplopen creek drawbridge and Fort Montgomery, 
18 to be seen a bed of crystalline limestone or calcite, filled with 
rough crystalline inclusions of an undetermined mineral, probably 
pyroxene, and much graphite. This is in all respects similar to 
those noted by Dr. Horton further to the northwest,” although this 
Particular outcrop seems not to have been observed by him. 
At the north end of the first cut above Fort Montgomery is a 
very curious narrow dike of dense black rock, four inches wide, 
traceable twenty feet or more vertically. It runs diagonally across 
the laminations of the gneiss, and seems to fill a well-defined crack. 
! Cf. Tenth Cen 
2 Geol. of N. apk ah a . 
