64 B. K. EMEHSON — DIABASE PITCHSTONE AND MUD ENCLOSURES. 



The appearance of the same layer at the base of the trap sheet is ex- 

 plained by the underrolling of the newly solidified surface of the sheet, 

 as when a carpet is unrolled on the floor, what was on top descends along 

 the front and comes to lie inverted beneath. 



Thus the porous mud-filled surface came to form, inverted, the base of 

 the bed, and to rest, though filled with fine mud, upon the coarse sand 

 onto which the sheet had advanced.* 



Diabase Pitchstone and Mud Enclosure. 

 origin of the pitchstone. 



This third process is still more peculiar and novel than the last. In 

 a tuff bed there are no caustic effects. In the overwashed and under- 

 rolled mixture of mud and trap the caustic effects are slight, and the 

 result appears equally in the upper and lower portions of the trap sheet. 



In the present case the caustic effects are at a maximum and take on 

 many of the aspects of metamorphism, and the work is normally con- 

 fined to the lower portion of the sheet. 



The flow of the submarine lava bed seems here to have been unusually 

 rapid, and the underrolling to have been a somewhat subordinate phe- 

 nomenon ; still the convection currents rising from the front of the bed 

 seem to have generally chilled it, so that a somewhat thin layer of com- 

 pact, heavjr, fine grained trap was solidified and underrolled to form a 

 basal bed protecting the liquid mass above. When the sheet had ad- 

 vanced over the muddy bottom so far that the imprisoned vapors could 

 not escape laterally, some slight and local disturbance broke up this 



*I have already reported very briefly upon this occurrence (Am. Jour Sci., vol. xliii, p. 147) ; 

 too briefly, it would seem, as the facts given were wholly misunderstood and incorrectly quoted 

 by Professor Dana and made to do duty in proof of the laccolitic origin of the Mount Tom trap 

 sheet. In his Manual of Geology, on page 805, he says : " The limestone had been torn off from 

 a layer not visible in the section." 



This was the very point I was trying to disprove, both by showing that there was no bed in the 

 older rocks of the region from which any such material could be derived, and that the shapes of 

 the enclosures were not such as would be possible in solid rock torn off from the walls of the fis- 

 sure through which the lava flowed, since it was iu thin filaments and flowed in to fill all the open 

 steam holes of the trap fragments. 



On the next page, 806, he says : "But a laccolilic origin and i.he abrasion of the underlying 

 sandstone are indicated by the occurrence of breccia beneath the trap, and especially by the lime- 

 stone chips in the lower part of the mass of the trap, and also over its upper surface, as described 

 by Emerson. A bed of limestone was evidently divided by the advancing tongue of melted trap, 

 part being left below and the rest above." As Emerson observes: "The facts prove that the 

 heavy trap flowed over the sandstone, abrading and tearing it." 



This was plainly quoted from a very dim recollection of the article in question. There is no 

 breccia beneath the trap. The inclusions cannot be called chips, and there is not the slightest 

 evidence that the melted trap has split asunder a bed of solid limestone. I have not made in the 

 article cited or elsewhere the observation quoted in the last sentence, since the facts all prove 

 exactly to the opposite. I know of no facts favoring a laccolitic origin of the Holyoke trap sheet. 



