hobbs.] STRUCTURAL GEOLOGICAL STUDIES. 17 
observations of his own respecting the topography and geology of the 
island. He appears to favor the view that the present configuration 
of the island has been brought about in part as a result of folding and 
in part by faulting. On page 36, he says: 
At the extreme western end of the Spuyten Duyvil Creek the opening of the 
Hudson suggests a crack, fault, or fissure; but east of this, at the cut of the railroad, 
the line of the creek seems to mark the delimitation of the limestone on the south 
from the gneiss on the north, and the creek has its bed in limestone, as the streams 
generally do in Westchester County (Dana). This limestone underlies the 
Harlem River and is produced in long prolongations underneath Fourth and Fifth 
avenues (at 132d street), and also under Eighth avenue, interrupted by gneiss, 
which appears to hold it in synclinal troughs. 
Gratacap apparently accepts the views of Dana regarding the fault 
at Manhattanville, and infers similar faults at Inwood (Tubby Hook) 
and along Spuyten Duyvil Creek. Stevens's assumption, and that of 
Dana and Merrill, that the Hudson has cut a deep gorge in a rock 
hypothetically considered as limestone, he considers an unlikely suppo- 
sition (p. 39). The' course of the Harlem River below 155th street he 
thinks probably is in a shortened synclinal trough (p. 40)." 
Julien. — As a result of a careful and thorough investigation of the 
dikes of basic igneous rock occurring on Manhattan Island, New 
York, Julien'' has produced a map which gives the orientation of the 
intrusions of basic- rock upon the island. The great majority of 
these strike practically parallel to the avenues or parallel to the Hud- 
son River shore, as will be clearly shown by inspection of his map. 
He says (p. 429): 
A linear arrangement of many exposures along the strike is apparent, but these 
were in most cases found to be interrupted without connection in the interspaces. 
As to indications of structure displayed by these diorites and horn- 
blende schists, Julien says that they testify at once to both extreme 
plasticity and extreme rigidity in comparison with the inclosing gneiss 
(pp. 430-482): 
First: Plication and corrugation of layers. The evidence of extreme plasticity in 
this rock during the general folding and kneading to which the strata of the island 
have been subjected is very markedly and frequently shown, not only by numerous 
folds with zigzag crumpling and distortion of the beds, but by corrugation of the 
layers even down to fine laminae. 
Second: Fracture and faulting. Evidence of extreme rigidity and brittlenesa are 
often also shown in the same beds in which corrugation and crumpling are promi- 
nent. These are crossed often abundantly by seams or veins of gray or white quartz, 
or of pegmatitic material, which run usually parallel to each other and at right 
angles to the foliation of the schist. * * * Such fractures plainly testily that the 
hornblendic beds have lain as rigid masses during the movements of the inclosin 
gneiss and have yielded only by rupture along the cross planes. 
a Descriptions ot special outcrops are given by Heinnch Ries in Trans. New York Acad. Sci., vol. 
10, 1891, pp. 113-114. 
''Julien, Alexis A.. Genesis of the amphibole schists and serpentines of Manhattan Island: Bull. 
Geol. Soc. America, vol. 14, 1903, pp. 421-494, pis 60-63. 
Bull. 270—05 2 
