THE HOLYOKE SHEET, 461 



At every point where the surface of the trap sheet can be inspected, 

 from where it crosses the Connecticut to where it crosses the West- 

 field-Holyoke Railroad, it has inehided a great number of fragments of 

 marly limestone and indurated clay, and the trap and limestone are often 

 kneaded together. Within the same limits the base of the trap repeats all 

 the peculiarities of the surface. It is araygdaloidal for about the same 

 thickness and in the same way; the same dove-colored limestone occurs 

 blended with the trap in the same way; and the subjacent arkose is 

 almost wholly unaffected by heat. The 300 feet of trap have not pro- 

 duced so much effect as is often seen upon the border of a 10-foot dike. 

 This is best studied at the river's edge at the north foot of Titans Pier. 

 On the other hand, where the molten surface of the trap sheet has come 

 in contact with the sands of the sea bottom, as at Titans Piazza, 100 rods 

 north, the trap is aphanitic at the contact, but pierced by great vertical 

 steam holes, and the sandstone is greatly baked. It seems that the broad 

 submarine trap sheet moved slowly westward, its incrusted surface being 

 covered by a fine marly clay deposit which was in places desiccated and 

 molded together with the still plastic trap, and that the surface was car- 

 ried forward to be rolled over the front and become the bottom along a 

 length of about 10 miles. The limestone and marlite inclusions of the 

 surface and base of the trap have been described in detail above and their 

 identity established, and similar cases of under-rolling of the Deerfield dike 

 and of the posterior dike have been given elsewhere.^ 



PETROGRAPHICAL DESCRIPTION OF THE NORMAL DIABASE. 

 GENERAL DESCRIPTION. 



The rock is so monotonously uniform in all its characteristics that much 

 repetition will be avoided by giving first a general description of the com- 

 mon type and then following this by a special discussion of the peculiarities 

 of separate occurrences. 



The rock from the "Iron Gate," or Thermopylae, where a passage has 

 been blasted through a projection of the Holyoke sheet for the river road 

 to South Hadley, near Titans Pier, coming from near the middle of the 

 sheet, is an especially fresh-looking variety, and may serve as the new type 

 for general description. 



> See pages 419, 470. 



