340 H. D. CAMPBELL AND W. G. BROWN — COMPOSITION OF TRAP. 



to be nearly the same. The large amount of magnesia (10.3 per cent.) in 

 G. H. Cook's analysis of the trap from the palisades of the Hudson is 

 worthy of notice. 



E. S. Dana* made a microscopic examination of numerous specimens of 

 trap from Connecticut, and of a few individual specimens from Nova Scotia, 

 New Jersey, Pennsylvania and North Carolina, and found that, so far as 

 microscopic structure goes, the rock from these distant localities is hardly to 

 be distinguished from the trap of the Connecticut valley. He gave the 

 composition of the rock to be " pyroxene, labradorite and magnetite, with 

 also occasionally some chrysolite and apatite." He, too, called it dolerite. 



G. W. Hawes t discovered a glassy ground-mass in certain modifications 

 of these rocks, and mentioned the occasional presence of biotite and horn- 

 blende. Excepting local modifications, he considered the rocks to be like 

 the ordinary old diabases, and in microscopic features to be monotonously 

 alike wherever fresh stones occur. 



J. P. Iddings X examined microscopically the igneous rocks occurring in 

 the earlier Mesozoic area at Orange, New Jersey, and found some of them 

 to be holocrystalline and others to contain glass. He says the former should 

 be called dolerites and the latter basalts. 



N. H. Darton,§ in speaking of the igneous rocks of the New Jersey Meso- 

 zoic region, says that they are remarkably uniform petrographically, as they 

 are all basalts, varying mainly in structure and development. 



These references are sufficient to show that the trap rocks of the earlier 

 Mesozoic areas upon the Atlantic border have been considered essentially 

 alike in mineral and chemical composition, whether called dolerites, diabases, 

 or basalts. 



Traps of exceptional Composition. 



Varieties. — AVe are glad to be able to bring to notice two interesting 

 varieties to break the monotony of these igneous rocks. On account of the 

 conspicuous occurrence of hypersthene in one variety, and of olivine together 

 with hypersthene in the other, we have called the former hype7'sthene-diabase 

 and the latter olivine-hypersthene-diabase. 



The palisade area of Triassic rocks extends from the Hudson river, through 

 New Jersey, Pennsylvania and Maryland, into Orange county, Virginia. 

 As early as 1839, W. B. Kogers|| called attention to the trap of the part of 

 this area lying in Virginia as being a very conspicuous feature from a geo- 

 logical point of view. He mentioned the occurrence of ridges, knobs and 



* Am. Jour. Sei., 3cl ser., vol. VIII, 1874, p. 390. 

 tProc. U. S. Nafc. Mus., 1881, p. 129. 

 i Am. Jour. Sci., 3d ser., vol. XXXI, 1886, p. 331. 

 § Bulletin U. S. Geol. Survey, no. 67, 1890, p. 15. 

 II Geology of the Virginias, 1884, p. 475. 



