Barrell — Groivth of Knowledge of Earth Structure. 139 



regions adjacent to the centers of learning were among 

 the first to receive attention and the Triassic basin of 

 Connecticnt and Massachusetts yielded information in 

 regard to the nature of igneous intrusion. This basin, 

 of unmetamorphic shales and sandstones, is occupied by 

 the Connecticut River except at its southern end. The 

 Formation contains within it sills, dikes, and outflows of 

 basaltic rocks which because of their superior resistance 

 to erosion constitute prominent hills, in places bounded 

 by cliffs. 



Silliman in 1806 3 described East Rock, New Haven, 

 Connecticut, as a whinstone, trap, or basalt, and 

 accounted for its presence on the supposition that it had : 



" actually been melted in the bowels of the earth and ejected 

 among the superior strata by the force of subterraneous fire, 

 but never erupted like lava, cooling under the pressure of the 

 superincumbent strata and therefore compact or nonvesicular, 

 its present form being due to erosion." 



In these conclusions Silliman was correct. With but a 

 limited amount of experience he was able to discriminate 

 between the intrusive and effusive rocks and saw that the 

 prominence of this hill was due to the erosion of the sedi- 

 ments which once surrounded it. 



An extensive paper on the geology of this region was 

 published by Edward Hitchcock in 1823, 4 then just thirty 

 years of age. This paper shows the evidence of exten- 

 sive field observations, and his comments in regard to 

 the trap and granite are of interest. Hitchcock gives 

 Hive pages to the subject of " Greenstone Dykes in Old 

 Red Sandstone" (6, 56-60, 1823) and makes the follow- 

 ing statements : 



"Professor Silliman conducted me to an interesting locality 

 of these in East-Haven. They occur on the main road from 

 New-Haven to East-Haven, less than half a mile from Tomlin- 

 son's bridge . . . (p. 56). 



They are an interesting feature in our geology, and deserve 

 more attention ; and it is peculiarly fortunate that they should 

 be situated so near a geological school and the first mineral 

 cabinet in our country ... (p. 58). 



3 Connecticut Academy of Arts and Sciences, 1810 ; quoted by G. P. 

 Merrill in Contributions to the History of North American geology, Ann. 

 Ept. Smithsonian Institution for 1904, p. 216. 

 _ 4 A Sketch of the geology, mineralogy, and scenery of the regions con- 

 tiguous to the river Connecticut; with a geological map and drawings of 

 organic remains; and occasional botanical notices, this Journal, 6, 1-86, 

 201-236, 1823; 7, 1-30, 1824. 



