812 DESCRIPTIVE GEOLOGY. 



trachytes, these diabases occasionally weather, on exposure, in a rough 

 cellular mass along the joints and fissures of the rock, which, however, does 

 not extend more than 6 or 8 inches below the surface. The southern- 

 most of these two hills is penetrated by basaltic dikes with sharp lines of 

 contact, and as they withstand atmospheric agencies better than the dia- 

 base, they form quite prominent ridges. The surrounding basalts are quite 

 like those already described; some of them, however, are porous, and others 

 have a peculiar brownish-red color diflticult to explain, but which, under 

 the microscope, is shown to be due to the color of the globulitic grains in 

 the glass base. 



Typical specimens of the diabase from Diabase Hills, and of the basalt 

 from the immediate neighborhood, have been subjected to chemical analysis 

 by Mr. R. W. Woodward, with the results as given below. In the first 

 and second columns are given the diabase, and in the third and fourth the 

 basalt: 



1. 2. 3. 4. 



Silica 54.52 54.80 53.94 63.98 



Titanic acid trace trace . . r . 



Alumina 19.10 19.10 17.05 17.05 



Ferric oxide.--. 2.83 2.67 2.93 3.00 



Ferrous oxide 5.89 5.90 7.15 7.09 



Manganous oxide trace trace trace trace 



Lime 7.25 7.26 7.41 



Magnesia- 3.92 3.78 4.67 



Soda 3.73 3.74 3.45 3.41 



Potassa 2.30 2.30 2.19 2.23 



Water - 0.59 0.62 i:iO 1.10 



100.13 100.17 99.89 



In both cases, the specific gravity varied from 2.6 to 2.7. 



These analyses agree very closely, and it is interesting to observe that 

 in their ultimate chemical composition, as well as in the individualized 

 mineral constituents, they should show such a remarkable analogy, where 

 the physical structure and geological relations of the two rocks differ so 



