THE NEWER CORES AND SHORT DIKES. 489 



THE TENTH CORE. 



The wood road that runs north into the mountain west of Moody 

 Comers branches after crossing the eleventh trap mass, and the western 

 branch in a few rods runs out on the sands of the large Glacial lake 

 described below. Here, at a pair of bars giving entrance to the field, 

 near a small brook, begins a long outcrop of trap, wliich continues 60 

 rods west, forming the bluff which made the south shore of the lake. 

 The coarse arkose surmounts it on the south, and it is by the downward 

 pitch below the sands of this sandstone on the east and the west tliat the 

 outcrop of the trap in those directions is limited, while the sands conceal its 

 northeiTi limit. Near its western end, where a stone wall runs across the 

 sands, at the foot of a marked bluff, the sandstone resting on the trap can 

 be seen to be well baked by it, and as the trap is wholly tine-grained and 

 without steam holes it is plainly intrusive. 



THE ELEVENTH OR BLACK ROCK CORE. 



Looking southeast from the Mountain House, on the top of Mount 

 Holyoke, one sees a prominent ridge of dark rock running parallel to the 

 mountain — indeed, duplicating it on a smaller scale, repeating its easy 

 southern slope and sharp northward-facing bluff and making with it the 

 great sweeping curve. It differs radically from it in its origin, the larger 

 deposit having been, as I have shown, a bed spread out over the subjacent 

 sandstone, and this an injected dike cutting across the latter. (See PI. 

 IX, p. 446.) This bluff, as seen from the mountain, is called "The Black 

 Rock," and I have chosen this name to designate the core, and also the 

 whole series of the newer trap intrusions. Seen from the west side of the 

 river above Smiths Ferry, it simulates exactly a volcano with sharp slopes 

 and central depression. 



The core is best studied at Batterson's quarry, in the northwest corner 

 of South Hadley, near the last house (E. H. Lyman's) before the town line 

 is reached. As seen in the accompanying view (PI. X), the nearly horizon- 

 tal sandstones are a remnant resting with their edges against the diabase. 



The latter not only cuts across the sandstone at this point, biit sends 

 into it apophyses of finer grain than the main mass, which have altered the 

 sandstones in places for 4 feet from the contact and have fused themselves 

 into finn union with the latter at their junction. The thin-bedded mica- 

 ceous sandstones are delicately plicated by the intruded trap. 



