276 TRANSACTIONS OF THE [may 20, 
the east shore of Blackwell’s Island a soft seam was met, and a lit- 
tle farther 10-12 feet of dolomite. Beyond the dolomite 50 feet 
of soft decomposed mica schist were cut, forming the shattered 
trough of asyncline on whose eastern side were 160 feet of white 
crystalline dolomite, precisely like tle outcrop at Kingsbridge. 
The eastern limit of this is charged with phlogopite. It was suc- 
ceeded by mica schist at first moderately solid. In thin sec- 
tion it is found to be thickly charged with pyrites, and to the 
decay and oxidation of this element of weakness is doubtless 
due much of the decomposition that marks so many bands of 
the schist. The mica schist soon gave way to another soft 
seam, 30 feet across, and like those on the New York side con- 
sisting of kaolinized pegmatite and greasy scales of chlorite— 
with lumps of quartz. Several minor alternations of rotten and 
solid seams intervened before firm schist was again encoun- 
tered. On the Ravenswood shore a massive hornblende gneiss 
or granite was met entirely different from any of the other 
rocks. In thin section it shows brown hornblende, quartz, 
orthoclase and plagioclase. 
The most interesting feature of this cross-section is the dis- 
covery of crystalline dolomite so far down the river. We have 
known of its presence under the Harlem river for many years; 
in fact, ever since Professor Dana foretold its probable discovery 
and the tunnel of the new aqueduct above High Bridge demon- 
strated it. The belt in the east channel opposite Ravenswood 
is, however, the prolongation of a belt that appears east of Mott 
Haven, unless faulting has thrown it to the east. It is also im- 
portant to note the close association of decayed pegmatite and 
chlorite with the limestone of the east channel, for it and similar 
associations elsewhere give us ground for believing that the kao- 
linized soft seams in the west channel indicate the former pres- 
ence of limestone there. Dr. F. J. H. Merrill at once pointed 
this fact out when looking over the writer’s specimens and notes. 
If so it must have been a shallow trough now worn completely 
away, but its presence would simplify the problem of the devel- 
opment of our local drainage lines on the east side of Manhattan 
Island; they would then be uniformly due to the relatively éasy 
erosion of limestone. There is thus some ground for thinking 
each river channel a compressed syncline, but from one isolated 
section like this it would hardly be justifiable to draw too lengthy 
conclusions about local stratigraphy. There is good reason to 
think the Ravenswood rock an intrusive hornblende granite or 
granite-diorite, with which it agrees in mineralogical composi- 
tion. : 
