THE UNDERROLLED MATERIAL. 63 



thick for all this ten miles, the same highly scoriaceous and mud-im- 

 pregnated stratum occurs inverted, presenting all the complex peculiari- 

 ties described above. The pores diminish and disappear upward in the 

 compact trap It is the same visibly crystalline trap as at the surface, 

 and not the very black aphanitic trap usual at the base, where it has cooled 

 in place, free from pores and enclosures. At the north base of Titan's 

 pier, at the water's edge is the best and most accessible place to study 

 the mud enclosures at the base, where the bed rests on coarse arkose. 

 Specimens taken from this place cannot be distinguished from those 

 taken from Dibbles crossing. At Titan's piazza, a hundred rods north, 

 along the base of the same sheet, the black compact aphanitic trap rests 

 on the same coarse sandstone, and contains only a few long steam holes. 



The same state of things exists at Larrabee's quarry, at the north line 

 of Holyoke, beside the Connecticut, at the top of the upper or posterior 

 sheet. A black carbonaceous shale is blended with the very porous trap 

 like a marble cake, causing its porosity, and sheets and films float in the 

 trap of shapes which could not have been torn from the rocks below 

 and floated up into their present place. Indeed, there are no such rocks 

 below in any of these cases. 



Just north of the quarry and across the road is a steep hill, and a fault 

 raises the trap in this hill so that its base is exposed, and contains the 

 same intermixture as the surface. As a very curious result of this mud- 

 and-water impregnation the trap also contains, deep in its mass, nests of 

 beautiful anhydrite, and the sandstone, just above, is filled with fine 

 hematite crystals, showing it was brought quickly over the trap while the 

 latter was still greatly heated. 



ORIGIN OF THE DEPOSITS. 



The following is offered as an explanation of these deposits : 

 The thick trap sheets have flowed out over the muddy bottom of the 

 bay, and their heat produced strong upward convection currents and 

 correspondingly strong indraughts from the sides, which carried muddy 

 waters out over the surface of the trap while it was still flowing, and 

 covered it with a quantity of calcareous mud, put of proportion to what 

 would have been carried in the same time by the normal currents. I 

 have seen sheets of newly solidified lava careen and slide beneath the 

 liquid mass at Kilauea, and the sheets of mud and lava may have thus 

 become variously mingled here, producing the results described above. 

 The. surface of the Holyoke trap sheet is filled with the fine mud just as 

 far north as the fine Chicopee shales extend, and farther north, where 

 the sheet flowed over coarse gravel, nothing of the kind occurred, be- 

 cause the coarse gravel could not be carried thus out over the thick sheet. 



