804 



HISTORICAL GEOLOGY. 



of large dikes, 180 to 300 feet wide, are shown in Pine Rock and Mill Rock, 

 on the map on page 299. Smaller dikes are very common in many localities. 

 The West Rock Ridge, Mount Tom Ridge, and Saltonstall Ridge afford 

 examples of outflow masses or sheets. With regard to the West Rock trap- 

 mass it is proved, on page 302, that it is a laccolith ; that the eruptive rock, 

 coming up from below, was forced into a space opened by itself between 

 layers of the sandstone, and there it accumulated under the weight of the 

 superincumbent sandstone, — probably one or more thousands of feet thick. 

 It is also shown that the upturned sandstone underneath the outflow, Tig. 

 1347, was profoundly abraded by the forced movement, over it, of the melted 



1347. 



View of the south front of West Eock, near New Haven, Conn., showing the columnar trap and the 



sandstone underneath it. 



rock, and thereby reduced to a nearly horizontal surface. No earth or stones 

 intervene between the trap and sandstone in the section exhibited, showing 

 that the material removed by abrasion was pushed on and lodged elsewhere ; 

 and also proving that the flow was not surficial, inasmuch as all surface 

 earth or debris is absent. It has been shown, besides, that East Rock, near 

 New Haven, is laccolithic ; and so also the trap belt next west of the Salton- 

 stall Ridge, and the second trap belt east of the same, as described by E. 0. 

 Hovey. In addition, the trap rests, in each case, on upturned sandstone, 

 proving that the upturning was a previous event for the region. It follows, 

 therefore, that the trap of the intervening Saltonstall Ridge must be similar 

 in mode of origin and time of eruption. 



