150 G. W. Hawes—Eruptive Rocks in. New Hampshire. 
ra of augite, a triclinic cee er titanic iron and minute 
rown dichroic crystals of hornblende. Some small amygda- 
loidal cavities were observed containing spheerosiderite, calcite 
and analcite. 
In these five closely adjoining dikes there are, therefore, four 
very different kinds of rocks. Selecting specimens as fresh as 
ssible from the different dikes I analyzed them with the fol- 
owing results :— 
Diabase. Olivine diabase. Diorite. Syenite. 
Dike No, 1. Dike No. 5. Dike No. 2. Dikes Nos. 3 & 4. 
Silica 41°63 42°77 41°94 58°25 
lumina 13°26 14°06 15 36 18°22 
Tron sesquioxide 3°19 2:32 3°27 1:07 
Iron protoxide 9°92 8°34 9°89 5°96 
Manganese protoxide "27 "15 "25 10 
Titanium dioxide 3°95 2°35 4°15 tr. 
Lime 86 11°47 9°47 151 
Magnesia 7°31 9°72 5°01 tr, 
Soda 2°49 1°89 5°15 4°19 
Potash 3°32 1°43 19 5°59 
Carbon dioxide 5°20 1°62 2°47 4°75 
Water 1°35 2°74 3°29 85 
100°75 99°26 100°44 100°49 
Though I have mentioned the existence of decomposition 
products, they are present only in minute quantities; and as In 
these very compact rocks the new compounds must have been 
formed from the old, I think the above analyses represent very 
nearly the original composition of the rocks, with, however, the 
addition of the water and the carbon dioxide. : 
Between the light and dark colored rocks there is a wide 
difference, which indicates that the reservoirs from which they 
were ejected contained fused material of very different compost 
tions. The black rocks are nearly alike in composition, but 
their differences are such as might account for the variation 10 
mineral constituents. The quantivalent proportion of the se- 
tion of olivine in one diabase and not in the other. But the 
presence of compact and porphyritic materials in different dikes, 
though of nearly the same composition, indicates different con 
ditions of cooling and crystallization, and these may also have 
n a cause of the mineral distinctions. 
In the adjoining Connecticut Valley the red sandstones are 
cut by numerous dikes. Many of these rocks and others ge 
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