120 BULLETIN OF THE 



needles and brownish glass. As regards their origin the microscopic 

 study yields no solution, but the field evidence leaves little doubt in the 

 observer's mind. The thin trap sheet overlying the lapilli is, wherever 

 observed at this locality, more or less vesicular, and in many places 

 cavernous. The greatest vesicularity is at its upper surface, and in the 

 hand specimens from the contact with the sandstone above the sand is 

 seen to have minutely penetrated the cavities and fissures of the scoria- 

 ceous amygdaloid. The sand grains not only occupy surface vesicles, 

 but they have percolated along cracks and irregularities in the trap to 

 a depth of two feet below the surface ; in some cases, they apparently 

 lie between or surround large areas of amygdaloid. Irving speaks of 

 similar phenomena in connection with the upper surfaces of extrusions 

 in the Lake Superior region, and refers to them as sandstone " veins." ^ 

 The lamination of the overlying sandstone is parallel to the surface of 

 the trap, conforming closely to its minor irregularities. Flowage action 

 is seen in the trap in the elongation of its vesicles. 



An interesting point is the occurrence of two thin layers of tufa in the 

 sandstone just above the trap, each about an inch in thickness and about 

 a foot apart. These layers appear in the hand specimen of a rusty brown 

 color, composed of water-worn fragments of trap mixed with clastic quartz, 

 and have a much weathered appearance. Under the microscope their 

 tufaceous character is well shown ; vesicular poi-phyritic trap grains 

 abound, and others of non-polai'izing character are derived from yellow- 

 ish glass, now partially or wholly devitrified. IMixed with the trap frag- 

 ments, there are abundant grains of quartz, muscovite, and orthoclase, 

 probably derived from the crystalline rocks which surround the Triassic 

 formation. The tufas as well as the sandstone effervesce readily with 

 dilute hydioohloric acid, owing to the presence of secondary calcite. 

 The sandstone owes its dark color in a large part to the presence of 

 comminuted dust-like particles of extremely weathered trap, scattered 

 through it and now altered to earthy chloi'ite and fine dots of ferrite. 



The several well-marked features of this interesting locality leave 

 no doubt tliat the trap sheet here is of extrusive origin. 



Hartford City Quarry. Locality 2G. — One of the posterior sheets, as 

 yet not safely correlated with other outcrops, forms a ridge of moderate 

 height, witli strong western blufi", in the southern part of Hartford, 

 where it is extensively quarried for road material. Trinity College 

 stands on its eastern slope. 



1 Copper-Bearing Rocks of Lake Superior, Monogr. V., U. S. G. S., 188.3, p. 292. 



