286 BULLETIN OF THE 



ever, a belief in overflows (240), though he quotes no decisive observa- 

 tion in this direction. Smith considered the trap eruptive over the 

 tilted layers of the secondary strata (225, 227), and Chapin represented 

 all the ridges about Wallingford as vertical dikes (fig. 8). 



Percival states that the traps " have obviously the character of intru- 

 sive rocks of igneous origin," which exerted " apparently a controlling 

 influence" in determining the arrangement of the sandstone (10, 11). 

 The trap occurs in dikes and ridges ; the dikes are small ; the ridges 

 are steep on one side, and on the other are frequently overlaid by sand- 

 stone, "thus apparently fox'ming interstratified masses or inclined dikes." 

 " In some instances, where the middle portion of a ridge appears thus 

 interposed, or merely as an overlying mass, its extremities appear as 

 distinct vertical dikes." "The ridge and the dike may thus be regarded 

 only as modifications of the same arrangement." (300.) His use of the 

 term "volcanic" (as 311) does not seem to imply that the eruptive 

 rocks ever reached the surface : nor does " cotemporary formation " 

 (299, 321) seem by the context to indicate the contemporaneous origin 

 of the traps as used in this article. He would apparently agree with 

 those who considered the trap eruptive after the making of the sand- 

 stone, often appearing between its layers and strongly affecting its 

 position. The younger Silliman at first (1842) thought the sandstones 

 tilted by the intrusion of the trap, but later reported to the American 

 association of geologists, in 1844, that the sandstone had been deposited 

 as now standing, and that the trap had been intruded without disturbance 

 and had seldom reached the surface. Whelpley held similar opinions. 



Professor Dana (6, 430) refers to Hitchcock's observations on the 

 back of Mount Tom, and adds, " But after an examination of the region, 

 the author regards it as more probable that the appearance of scoria is 

 owing to an escape of steam laterally from between the opened strata 

 during the ejection of the trap of the adjoining mountain." In the later 

 editions of his Manual, he makes no mention of overflows. He states 

 that the trap " has come up through fissures in the sandstone which 

 varied from a few inches to three hundred feet or more in breadth. In 

 many cases, it has made its way out by opening the layers of sandstone, 

 and in such cases it stands with a bold front, facing in the direction 

 toward which it thus ascended." (c, 419; also d, 46.) "The manner 

 in which the trap at its eruption has sometimes separated the layers of 

 sandstone, and in this way escaped to the surface, instead of coming up 

 through the fissures simply, shows that the rock had been tilted exten- 

 sively before the ejection." (c, 421.) 



