460 GEOLOGY OF OLD HAMPSHIEE COUNTY, JIASS. 



The thick trap sheets flowed out over the muddy bottom of the bay, 

 and their heat produced strong upward convection currents and corre- 

 spondingly strong indrafts 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 out of proportion to what would have 

 been carried in the same time by the normal cun-euts. 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 fine mud just as far north as the fine Chicopee 

 shales extend; and farther north, where the sheet flowed over coarse 

 gi-avel, nothing of the kind occurred, because the coarse gravel could not 

 be thus carried out over the thick sheet. 



ON THE UNDER-ROLLING OF THE SOLIDIFIED SURFACE OF THE TRAP. 



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

 explained by the under-rolling 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.^ 



'I have already reported very briefly upon this occurrence (Am. Jour. Sol., 3d series, 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 laccolithic origin of the Mount Tom 

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

 layer not visible in the section." 



This was the very point I was trying to disprove, by showing both 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 

 inilusions were not such as would be possible in solid rock torn oft' from the walls of the fissure 

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

 holes of the trap fragments. 



On the next page, 806. he says: "A laccolithic origin and the abrasion of the underlying sand- 

 stone are indicated by the occurrence of breccia beneath the trap, and especially by the limestone 

 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, jjart 

 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 plaiuly quoted from a very dim recollection of the article in question. There is no 

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

 evidence that the melted trap has split asunder a bed of solid limestone. 1 have not made, in the 

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

 the opposite. I know of no facts favoring a laccolithic origin of the Holyoke trap sheet. 



