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



Quartz was noticed under the microscope in the rock from the " Dump- 

 ling " as isolated grains with fluid inclosures, and also intergrown with the 

 feldspar, showing granophyre structure. 



Green hornblende has been noticed in several sections. 



Description of the Olivine- Hypersthene-Diabase. — This rock has a coarser 

 grain, a more greasy luster and a darker color than the one described above- 

 It weathers into globular masses. In thin sections the crystals of hypers- 

 thene are conspicuously larger than in the other rock, some of them meas- 

 uring 6 mm. in length and 3 mm. in width. Olivine occurs quite abundantly 

 as fresh crystals, with well defined outline, varying in size from 0.2 mm. to 

 4 mm. in diameter. Alteration has begun to take place along only the 

 cracks. Biotite seems to be more common in this rock than in the hypers- 

 thene-diabase. The other constituents of the latter rock make their appear- 

 ance here also, with perhaps the exception of quartz, which we have not 

 observed. 



The composition of this rock and its relations to the hypersthene-diabase 



as well as to a normal Mesozoic diabase from Connecticut are shown in the 



following analyses : 



Chemical Analyses of Diabases. 



I. — Hypei'sthene- II. — Olivine- III. — Diabase, 

 diabase. hypersthene- West Roek, 



diabase. Neiv Haven. 



Sp. gr. r= 3.09. Sp. gr. = 8.10. Sp. gr. = 3.03. 



SiO., 51.31 50.88 51.78 



A1203 13.64 13.17 12.79 



FejOg 0.52 1.11 3.59 



FeO 8.49 9.66 8.25 



MnO trace trace 0.44 



CaO 12.41 10.19 10.70 



MgO 12.73 13.05 7.63 



KgO 0.32 0.31 0.39 



Na.^0 1.40 1.17 2.14 



TiOg trace 1.41 



P2O5 trace 0.14 



Ignition 0.14 0.63 



100.82 99.67 99.89 



The material chosen for analysis was perfectly fresh. The hypersthene- 

 diabase (I) came from the quarry on the " Twins," in Culpeper county, 

 Virginia. The olivine-hypersthene-diabase (II) came from a dike in the 

 railroad cut not far from the " Twins." 



We wish to call attention to the occurrence of hypersthene, a mineral rich 

 in magnesia, in a diabase (I) which contains a large percentage of magnesia 

 (12.73 per cent.) in contrast with the absence of this mineral in the normal 

 diabase (III), which contains a very much smaller percentage of magnesia 

 (7.63 per cent.). The olivine, together with hypersthene, also occurs here in 



