MUSEUM OF COMPARATIVE ZOOLOGY. 105 
3.— General Features of Intrusive and Extrusive Sheets in 
Connecticut. 
GROUP I. WESTERN RIDGES. 
Locality 1. Section numbers, 85-89. Local name, East Rock. Percival’s Report, 
pp. 895-398. Percival’s notation, W. S. I. (1). 
General Account. — The southwestern face of this fine mass is well 
exposed in a strong palisaded cliff on the border of New Haven, below 
which the underlying sandstones can be seen at several points. The over- 
lying sandstone close to its contact with the trap was found on the 
northeastern slope, in the woods, about a third way down from the sum- 
mit. This rock is regarded as a part of the West Rock sheet, from 
which it is thought to have been separated by a fault ; similar faults 
of smaller throw are supposed to account for the notches in the south- 
eastern extension of East Rock itself. 
Sections cut from specimens taken from the upper contact and from 
four feet below it cannot be distinguished from sections similarly selected 
from the base of the sheet. 
The trap is wanting in vesicles of expansion throughout its mass, and 
is holocrystalline except at contact with other rocks. Extremely close- 
grained and glassy at the upper contact, where it shows microscopic 
flowage parallel to surface of junction with the overlying sandstone. 
Sandstone directly above does not contain fragments of trap; band 
specimens appear much more dense than from beds distant from the 
trap sheet. 
Locality 2. West Rock. Percival’s Report, pp. 394-396. Percival’s notation, 
W.S. I. (4). 
The general features of this ridge are like those of East Rock ; but 
no exposure of the upper contact has been found on its back. As far as 
seen, it is of dense texture, even in the uppermost parts exposed. The 
southern end of the ridge, where the underlying sandstone is quarried 
and exposed in contact with the trap, may be reached by the West 
Haven horse cars from New Haven; the remainder of the ridge is 
wooded and less easily examined. 
Locality 3. Section numbers, 194-199. Gaylord’s Mountain, Roaring Brook. Perci- 
val’s Report, pp. 402-404. Percival’s notation, W. S. LI. 
Gaylord’s Mountain is a slightly dislocated continuation of the West 
Rock range; on its back, Roaring Brook has cut a picturesque ravine, 
