WHITE DIABASE TUFF. 61 



diabase of the region, but containing no iron and possibly a very few 

 colorless pyroxene crystals. 



In the interior the rock looks like a white silicious limestone. On the 

 outside weathering empties the small round amygdules which are lined 

 by perfect secondary plagioclase crystals radiating inwardly and the 

 center is filled with calcite, in the middle of which is generally a grain 

 of pyrite. It occurs a few feet above the Holyoke trap sheet and is the 

 oldest tuff in the valley. It was derived, I have no doubt, from some 

 portion of the surface of the Holyoke sheet, and is one of the abnormal 

 forms produced by the agencies described below. I have found some- 

 what similar forms at Dibbles crossing and the secondary feldspars en- 

 crusting cavities are common at Greenfield. 



Mud Enclosures. 

 general character of the surface material. 



See plate 3. 



At Dibbles crossing, on the south line of Hotyoke, at the south end of 

 Ashley reservoir, is a most peculiar facies of the trap. The sandstones 

 are stripped from the upper surface of the Holyoke trap sheet over a 

 broad area, and two railroad cuts, convenient to the roadside, enable 

 one to study the fresh rock. 



For a depth of 12 or 15 feet from its upper surface the trap is filled 

 with a pearl gray compact calcareous clay rock of just the appearance of 

 a common clay concretion or with a thin bedded fine grained gray sand- 

 stone. It sometimes makes the impression of claystone fragments en- 

 closed in trap, sometimes of trap fragments enclosed in claystone. The 

 two are often intimately mixed together like two nonmiscible fluids, and 

 the dark gray or red brown trap and the pale gray clay rock produce 

 the effect of Castile soap. Long filaments and stringers and rows of 

 bubbles of the clay go out very generally from the larger areas of the 

 clay rock into the trap in a way only explicable on the assumption that a 

 mass of the muddy clay was thrust suddenly into the liquid trap. In 

 this same clay mass w r ill often be found many small angular fragments 

 of the amygdaloidal trap, showing that the trap was partly solidified 

 and was then shattered by explosions caused by its contact with the 

 liquid mud. At the north end of the east wall of the cutting is a sheet 

 of the clayey sandstone, which is about twelve feet long, a foot wide at 

 the center, and tapering to nothing at the ends. Above and below the 

 trap is coarsery amygdaloidal or rather abounding with rounded, beaded, 

 and variously lobed cavities, which are filled with the gray mud.* Some 



* The later infiltration of calcite has changed this mud into a massive gray rock exactly like 

 the claystones so common in the Champlain clays. 



