498 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



sheets, and then a second period of volcanic activity followed. The lav;i 

 broke out at a point on the fissure through which the older lava had come, 

 and flowed in a broad sheet down the bottom of the valley. The place 

 where the lava came up is preserved in Little Mountain, east of Mount 

 Tom, and the outcrop of the lava sheet extends north and south from here, 

 and has been described as the posterior sheet. 



The next episode in the history, following immediately on this outflow 

 of lava, was an explosive eruption of diabase forming the beds of tuff from 

 the Belchertown ponds to Holyoke. The center of eruption seems to have 

 been at the focus mentioned above, not far from Smiths Feny, since the 

 bombs there are a foot across and decrease in size in botli directions. The 

 tuffs produced a marked change in the fucoidal ^ beds (the Longmeadow 

 sandstone) above, into which they grade, as compared with the older beds 

 beneath the tuff. By shallowing of the water tlie beds are rendered finer, 

 and they are made deep-red and calcareous from the decomposition of the 

 tuff. These red beds extend south from the lunate band of tuff, but this 

 tuff only accelerated and intensified a process which extended far south 

 beyond its influence, and which had its cause in the width of the basin, its 

 shallowness, and the presence of northward currents along its west side 

 and southward currents along its east side. These cun-ents kept the sides 

 of the basin deeper and made them a seat of coarser sedimentation, and 

 between them was a central area of conflicting and shifting currents — a sort 

 of Sargasso Sea, in which the finer fucoidal sandstones were deposited and 

 so frequently exposed by the retreating tides that almost every part shows 

 mud-cracks, rain-drops, tracks, thin films of coal, or some trace of exposure. 



The northern or Montague basin reached only this stage, the broad 

 development of the central fucoidal sandstones, and there was no tuff 

 outburst or development of shales. In the wider southern or Springfield 

 basin a central area of still greater (piiet developed with the widening 

 of the channel, in which at last marly sediments were retained within 

 the area, marked by numerous salt pseudomorphs, and in which a later 

 circulation of the waters has concentrated the lime into bands of concre- 

 tionary limestone. 



'These paragraphs were written when the rodlike markings in the "fucoidal" sandstones were 

 supposed to be plant remains. I now think them to be concretions, as explained under "Plants," p. 

 395. Tbi're may thus be added to the effects of the impregnation with iron the abundant rod-shaped 

 concretions which have been mistaken for plant remains and called "fucoids," 



