the Earth's Contraction from Cooliny . 279 



ore in disseminated grains or crystals. Analyses have shown 

 that the trap of this region in Connnecticnt and New Jersey is 

 essentially identical in constitution; and an examination and 

 determination of the density of specimens of the same rock from 

 North Carolina and Nova Scotia leaves no doubt that it is funda- 

 mentally alike throughout. The aspect of the ordinary speci- 

 mens from these widely distant regions is closely the same ; and 

 the density varies little from 3. The dolerite of East Rock, 

 New Haven, Conn., gave Professor G. J. Brush for the specific 

 gravity 3*09-3'085 ; from West Rock, ib., 3*045-3'07 j from 

 the vicinity of Raleigh, North Carolina (a specimen received by 

 the writer from Professor Kerr, of Raleigh, and not distinguish- 

 able by the eye from an East or West Rock specimen), 3' 16. 

 Professor Cook, of New Jersey, obtained for the dolerite of the 

 Palisades (Geol. Rep. N. J. p. 215) 2-94. Professor H. How 

 gives the writer, for a similar variety from Nova Scotia, 2*94. 

 Complete analyses have not been made of many of these rocks. 

 The following are analyses of the West-Rock (New Haven) do- 

 lerite and that of the Palisades, New Jersey (seventy miles west 

 of West Rock) — -the former by W. G. Mixter, Assistant Chemist 

 in the Sheffield Scientific School, and the latter by Professor G. 

 H. Cook, of the New Jersey Geological Survey : — 



SiO 2 . 2A10 3 . FeO. MgO. CaO. *rg Ign. 



1. West Rock. 5237 1231 13-13 603 1074 2-62 0'94= 98-14 



2. Palisades t 539 J7'5 8'0 10-3 80 [2*3] .. =100. 



There are variations in the dolerite of the regions above re- 

 ferred to depending on the proportion of the felspar and also on 

 the presence of water ; but they occur in various parts of the 

 several regions instead of characterizing any one of them. The 

 dolerite of dikes in East Haven, Conn, (the town adjoining New 

 Haven on the east), diminishes in lustre as the dike is more 

 remote from the New-Haven line; and in the ridges near Sal- 

 tonstall Lake^ two miles distant, it is faintly glistening and of 

 somewhat less hardness ; and although generally solid through- 

 out, it is in some parts vesicular (amygdaloidal). A precisely 

 similar rock in all its characters occurs among the trap-hills 

 (dolerite) of Meriden, called the Hanging Hills; and part of it 

 is amygdaloidal. The same is found also in Nova Scotia. An 

 examination of a specimen from the vicinity of Saltonstail Lake, 

 by Professor O. D. Allen, shows the presence of 4*53 per cent, 

 of matter driven off on ignition, after deducting the hygroscopic 

 moisture (0'6G). It indicates the presence of some hydrous 

 mineral (chlorite apparently) in place of part of the augite or 

 felspar; but the percentage of silica is 49'88, 50*10, according 

 to two determinations by S. T. Tyson, of the Sheffield Scientific* 



