l66 MINERALOGY AND LITHOLOGY, 



with the proportion of the ingredients from light gray to black. Future 

 search will certainly bring to light many more varieties of this rock than 

 I have to describe, for the regions in which these rocks are found are 

 strown with boulders which are composed of varieties of gabbro, which 

 it would be a pleasure to investigate if their source were accessible. 



In Silesia, and in some other regions, mountains and cliffs composed 

 of gabbro are conspicuous in the landscape. Owing to their smaller 

 bulk, and the condition of the surrounding strata, this is not the case in 

 New Hampshire ; but as lithological specimens, they are no less interest- 

 ing. There are some general differences between the specimens from 

 our two localities where this rock is found in place. We will first speak 

 of the gabbro from Waterville. 



As here found, the rock is very coarse in texture, and nearly black. 

 The feldspar is dark in color, and possesses bright cleavage surfaces on 

 which fine striations are very conspicuous. In regard to the chemical 

 and microscopic properties of its individual minerals, considerable has 

 already been said. Analyses of the feldspar and olivine, by Mr. E. S. 

 Dana, have been given on pp. 93 and 70. These analyses show that 

 the feldspar is labradorite rich in lime, and that the olivine is a variety 

 very rich in iron. The diallage, though apparently black, is in thin sec- 

 tions, of a pinkish color. This mineral, in many typical gabbros, when 

 cut and examined with polarized light, presents a very fine fibrous struct- 

 ure. This is not seen in the diallage of our rocks, and neither is it at 

 all an essential feature, since the ready separability of the augite into 

 laminae is the characteristic of diallage, and the fibrous nature of the 

 mineral in thin sections is only characteristic of occurrences from certain 

 localities. The augite of our rocks is like that of the variety that has 

 been called palatinite. This name was given by Laspeyres to a gabbro 

 of carboniferous age, which is abundant about the Pfalz on the Rhine, 

 though varieties from other regions have since been embraced in the 

 name. This gabbro, however, is not entirely crystalline, but contains 

 more or less of glassy matters which are not found in our rocks. The 

 unessential nature of the distinction between diallage and augite, and 

 the identification of all the intermediate structural varieties between the 

 most typical specimens of the two minerals, makes more forcible what 

 was previously said in regard to this rock, — that when strict rules are 



