THE POSTERIOR SHEET. 465 



the impression that it extends, tit least in a fragmentary wa^-, tar east. The 

 long- eastern projection of the Black Rock plug (see PI. IX, p. 446) and the 

 one east, and the string of smaller i)lugs elongate east and west, seem to 

 be parts of it. They are, however, true intrusions, and their elongation 

 seems rather to indicate the existence of a common ancient and deep-seated 

 fissure through which they have been extruded. This is proved by the 

 fact that they cut directly across the beds of the sandstone below the tuff, 

 the tuif itself, and the sandstone above, while west of the river the tuff 

 rests directly upon the posterior sheet. 



The sheet appears first as a great reef projecting into the Connecticut 

 a mile below Mount Tom station, its northern portion fine-grained and col- 

 umnar, its southern coarse and in great blocks, and is doubtless continuous 

 beneath the sand southwest to the interesting outcrop at Lymans Crossing 

 (the first crossing below Mount Tom station), where a wall of trap is exposed 

 in the railroad cut. The northern portion of the cut is rudely columnar 

 trap, with an irregular surface dipping about 35° SE. Resting upon this 

 surface is a coarse trap agglomerate, consisting of blocks a foot across and 

 a fine sandy paste, in which many flakes of grapliite appear. This is the 

 normal relation of the tuff to the posterior sheet for a long way south. 



A rod south of this tuff is an outcrop of trap which, from its great fresti- 

 ness and compactness, and from its containing inclusions of coarse amyg- 

 daloid from the tuff, I associate with the Burnt Mill plug just south, which 

 interrupts the sheet at this point. A few rods south of the crossing a brook 

 crosses the road, and on it is the ruin of Aldrich's leather mill, burnt many 

 years ago. The brook flows east along the course of a transverse fault, and 

 at and below the dam can be seen very finely the outcrop of an intruded 

 ti'ap mass, which clearly cuts across the sandstones, bakes and twists them, 

 and extends west along the north side of the mill pond. (See p. 41)4.) 



South of the brook and the fault the outcrcips are continuous, and the 

 posterior sheet can be seen to be wholly indei)endent of the core which crops 

 out north of the stream at the dam. Commencing at the railroad culvert 

 over the brook, the sandstone can be seen on the south side of the brook in 

 contact with and beneath the trap of the posterior sheet and having the 

 unusually steep dip of 60° SE. beneath the trap because of the fault. From 

 this point the sandstone can be followed along the south bank of the brook 

 continuously, past the mill and the pond. It dips regularly to the southeast 



MOW XXIX .30 



