THE HOLYOKE SHEET. 451 



is present on this side, sandstone occurs in immediate proximity to compact 

 trap for long distances. On the west, in many cases, if not in all, vertical 

 bluffs and "Devil's Gardens" of trap ddbris coincide with the fault bound- 

 aries of the trap along the uplifted edge of the blocks. The researches of von 

 Koeneu^ as to recent movements on such fault planes suggest the possibility 

 that many of these vertical trap bluffs may be the result of such recent 

 movements. I think this consideration has sufficient force to dejirive these 

 vertical bluffs of any value as measures of the time since the disappearance 

 of the ice, as I have attempted to use them elsewhere. The effect of these 

 faults is more manifest upon the narrow posterior bed. (See p. 473.) 



The results regarding Triassic faulting are in accord with the very 

 valuable discoveries of Prof. W. M. Davis in Connecticut (p. 377.) So far 

 as the substratum beneath the Triassic is regular and has north-south strike, 

 the faults agree therewith. Where, under the Mount Holyoke range, the 

 substratum is a great granite massive and two great trap plugs further 

 complicate matters, the faults are correspondingly irregular. 



GENERAL CHARACTERISTICS OF THE SHEET. 



At the east end the bed is amygdaloidal in almost its entire thickness, 

 and greatly decomposed. This general decom])osition of the whole mass 

 is a striking characteristic of the whole bed, and even where it seems as 

 fresh as possible, as where it was blasted through at the "Iron Gate" (Ther- 

 mopylse) for the passage of the riverside road to Soutli Hadley, the micro- 

 scope shows it to be deeply decomposed. It presents far less range and 

 variety of texture than the Deei-field bed, being mostly aphanitic and 

 showing only a faint porphyritic structure by the development of the 

 earlier generation of feldspars to di.stinct -sdsibiHty. Back of the Holyoke 

 Mountain House and on Titans Pier it is exceptionally coarse-grained and 

 gabbro-like in texture, the broad, flat, black sheets of pyroxene being often 

 markedly warped and one-half inch in length. Unlike the newer traps, its 

 fissures are cemented by quartz. 



Following the sheet westward the amygdaloidal texture is confined to 

 the upper portion of the bed, except where it is luider-rolled, when a marked 

 steam-hole structure takes its place at the base of the bed. 



It is everywhere, after reaching its full thickness, rudely columnar, and 

 at Titans Piazza the columns are of the largest size and in great perfection. 



' Jahrbuch K. preuss. geol. Landesanstalt, 1886, p. 467. 



