MUSEUM OF COMPARATIVE ZOOLOGY. 12k 
The upper portion of the sheet is vesicular, but its upper contact is 
not seen. The under contact is well revealed in the quarry, and affords 
the best exposure for the study of the base of a sheet that we have 
yet found. It is of interest also as being the locality described many 
years ago by the elder Silliman.’ Yet this particular contact is not 
altogether characteristic of the under contact of most of the extrusive 
sheets, for as a rule the junction of the trap with the shale is without 
complication of any kind: one lies smoothly on the undisturbed sur- 
face of the other. 
_ The underlying shale of the ‘quarry will be first considered. Four 
‘inches below the trap, the shale locally consists of tufaceous mate- 
rial. Round and linear fragments of yellowish brown glass are seen 
under the microscope, thickly sprinkled with minute particles of some 
decomposition product of iron. These partially devitrified glassy areas 
are undoubtedly the remains of obsidian-like fragments deposited as the 
normal result of erosion from some volcanic flow, or as ejected matter 
from a voleanic vent. In either case, volcanic vents sent forth showers 
of ashes or flows of lava, presumably at no great distance from this 
point, and at the time of the deposition of the sandstone. 
The contact line between the bottom of the trap and the under- 
lying shale is as a rule irregular and indistinct. The lower portion 
of the trap for a distance of four feet presents a very vesicular and 
scoriaceous appearance, not unlike the upper surface of the lower flow 
exposed in the Meriden Quarry. The microscope shows portions of this 
scoriaceous material thickly sprinkled with well marked gas cavities, 
many of them having a linear arrangement, roughly parallel to the 
upper surface of the shale, due to the flowing action of the trap while 
in a viscous condition. The same parallelism is also well shown at the 
upper surface of the first flow in the Meriden Quarry, locality 19. 
The trap for a thickness of several feet is not only abnormally scoria- 
ceous, but is extremely broken. Irregular and rounded areas of vesicular 
trap are apparently cemented together by brown calcareous sandstone 
possessing a lamination generally parallel to the stratification of the shale 
below. The microscope shows these brown areas to be mixtures of 
secondary quartz, calcite, and a little chlorite, arranged in layers ; they 
must have been deposited by infiltrating waters. The texture of the 
trap gradually increases in coarseness as we approach the central part 
of the sheet, and then grows porphyritic and finer-grained near the up- 
per surface. Careful search has failed to discover its upper surface in 
1 Amer. Journ. Science, X VII., 1829, pp. 121-132. 
