MUSEUM OF COMPAKATIVE ZOOLOGY. 121 



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 afibrds 

 the best exposure for the study of the ^xiae 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 tufixceous mate- 

 rial. Eound 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 volcanic 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, XVII., 1829, pp. 121-132. 



