SUMMARY OF HISTOKY OF THE TKIASSIO BEDS. 497 



wlu^lly removed from the basin during the dejjosition of beds of so great 

 tliickuess. At a ceitain time in the midst of this rapid deposition came 

 the great eruptions, apparently synchronous, of the Deerfiekl and Holyoke 

 beds. The bottom of the bay at this time presented a surface covered in 

 different parts b)' beds of every degree of coarseness akmg its l^orders and 

 grading toward the center into finer beds, as is indicated by the character 

 of the substratum. on which the trap beds rest. 



Along beneath the trap of the Holyoke range fi-om west of Mount 

 Holyoke to beyond Mount Tom much argillaceous limestone is inclosed in 

 the trap at its base, and in Holyoke, at Ashley's pond, the same limestone 

 occurs in the inclusions at the surface of the same trap sheet. 



I explain the above structure by the under-rolling of the suiface of 

 the sheet. A limited amount of calcareous mud was washed by the strong 

 con\'ection currents onto the submerged surface of the advancing sheet 

 (which was superficially solidified) and blended more or less with tliis sur- 

 face, which by the continued advance of the mass became in part under- 

 rolled, like the smi'ace of an unrolling carpet, thus protecting the sand 

 below from baking, and bringing the highly vesicular trap loaded with 

 limestone to the base of the bed. A similar structure occurs at the base of 

 the trap sheet east of Greenfield, but not at its surface. This was caused 

 b}' the frothing up of the muddy bottom into the liquid trap. 



It is quite probable that these trap sheets were poured out through 

 fissures — the Holyoke sheet through an east-west fissure passing beneath the 

 line of small volcanic cores a mile south of the main outcrop of the main bed, 

 and continuing as a north-south fissure passing through the same series of 

 plugs west of the Connecticut. The focus of most intense and long-con- 

 tinued action was about a mile south of the point where the ri^^er cuts 

 through the main ridge and where these fissures intersect. The Deerfeld 

 fissure can not be exactly located. It was beneath or east of the present 

 outcrop, because artesian wells at Turners Falls cross the trap sheet just 

 east of this outcrop, and there is no trace of intrusive trap west of the 

 present western blufi's. Its focus was probably just east of Greenfield. 

 These sheets produced no disturbance in the distribution of sediments and 

 almost no tuffs, and the arkose which covers them is often buff and nearly 

 free from iron and lime. 



The sands which spread over the basin soon covered the great trap 

 MON XXIX 32 



