FOOT-TRACKS AND TRAP SHEETS. 379 



directly on the coarse arkose, is filled with fragments of the fine-g-raiued 

 shales and dove-colored limestones which were in place as part of the bot- 

 tom far to the east or southeast. This shows that the fissure was situated 

 east or southeast of the present outcrop, and that the trap broke through 

 and flowed out, first over the mud flats and then over the coarse granitic 

 debris lying westward. The greater thickness of the trap sheet in just this 

 portion of its length, viz, in Mount Tom, may Ije because the trap slieet 

 extended into the deeper shoreward portion of the l)asin — that occupied 

 by the western, northward-moving current. 



THE POSSIBLE CONISTBCTION OF THE FOOT-TRACKS WITH TIIE TRAP 



SHEETS. 



It is furthermore interesting to observe that all the famous localities of 

 ti'acks are far out in the center of the ancient bay, in sandstones that rest 

 directly upon the back of the broad trap sheets, and not very high u]) above 

 the upper surface of the trap. 



Above both the Deei-field and the Holyoke trap sheet the area within 

 which these tracks occur is approximately identical with the area overspread 

 by the trap sheet, and it seems to me quite probable that the shallowing of 

 those broad central areas of the bay 300 to 400 feet by the great trap sheets 

 may have produced the peculiar sui-faces just between tides, on whose sand 

 and mud flats the reptiles walked and the raindrops made their marks. The 

 iron which was soon set free from the decomposing lavas below permeated 

 the muds and, besides giving them their red color, cemented them Avith 

 unusual rapidity, and so favored the very remarkable pi-eservation of the 

 tracks, as tlie preparation of the broad central intertidal mud flats favored 

 their production. 



There are more than 20,000 tracks in the Amherst collection, perhaps 

 as manv more in that of Yale, and again as many more in other collections, 

 and it is hard to say how many have been destroj'ed for every one in the 

 collections. Tliere is, therefore, something quite exceptional to be explained 

 in the vast number of these tracks which are found in this very limited 

 space. There is a slight possibility that the heat of these great trap sheets 

 may have jwomoted rapid consolidation of the sand layers by which they 

 were quickly covered. 



