THE NEWER CORES AND SHORT DIKES. 491 



the back of the main diabase bed, rising gTadually north to the Holyoke 

 House, its left or south side the vertical wall of the Black Rock dike, its face 

 veneered to a varying distance upward with the remains of the sandstone. 



When one comes out where one can look down on the cleared sand tlats 

 of the post-Glacial lake mentioned above, one sees thattlie boundary of the 

 Mount Holyoke bed continues east, while that of the great crater swings 

 round southeast and extends to the deep gorge of the little brook which 

 drains the basin of the lake above mentioned, and has cut deeply through 

 the diabase to enter Elmer Brook, just north of H. White's. The diabase 

 continues to rise high and to carry a thin remnant of the sandstones in con- 

 tact with its vertical face, which sandstone shows contact effects and can 

 often be plainl}- seen to abut against and not to underlie the volcanic rock, 

 toward which it dips. WHiere the boundary of the diabase runs southeast 

 the sandstone preserved its east-west strike in the main, but in jjlaces dips 

 toward the diabase with the abnormally high angle of 80°. 



At the west end of the mass the diabase appears in the road at the 

 Lyman house, and its westward extension is concealed by sands. This is 

 also the case with its southern border. The outcrop at the point where 

 Elmers Brook crosses the road is so brecciated and its fissures are so filled 

 with druses of small rhombohedra of hematite that it is probably near the 

 southern contact. 



On following the southern edge along to a point about north of the 

 schoolhouse, where the road to South Hadley starts, it is seen that sand- 

 stones appear on the south of the trap, sti-ike N. 65° E., dip 15° S. — fine- 

 gi'ained, calcareous sandstones, blue-black as if baked or loaded with vol- 

 canic ashes, and rusting slowly inwardly, like the diabase, and between 

 them and the diabase is a band, apparently 10 to 15 feet wide, of the most 

 perfect tuff, made up wholly of angular trap fragments of the size of a pea, 

 with here and there one as large as an acorn, all greatly decomposed. The 

 exact relation of the tuff to the other beds could not be made out. 



The boundary can be closely followed eastward to the brook east of 

 White's wood road, to which the northern boundary has already been 

 followed. The exact contact can not be seen, but the dark rusting sand- 

 stones dip south away from the diabase, while the latter rock in the imme- 

 diate proximity to the contact (6 to 8 feet distant) is compact, coarse- 

 grained, and not porous. 



