SECOND AND THIRD VARIETY OF GABBRO. 271 



has certain features connecting it more closel}^ with the pyroxene-granu- 

 lites. 



SECOXD' VARIETY OF GABBRO. 



Area of Occurrence. — A second variety of gabbro occurs on botli sides 

 of the road about midway between Oswegatchie Settlement and Diana 

 Center. The field relations of the rock are not so clear as in the case 

 just described, but they indicate an intrusion of no very great extent, 

 cutting the third variety of gabbro, to be considered below. 



Description of the Rock. — The rock may be sufficiently described by the 

 statement that both in the hand specimens and in microsections it is 

 practically identical with the hypersthene-gabbro of southern Hamil- 

 ton county described in the paper referred to above. 



Relations and Origin. — As the two localities are about 60 miles apart, 

 it would seem at first sight rather strange that there should be such 

 identity in the character of the rocks, which occur in quite inconspicuous 

 masses. The explanation of the fact doubtless lies in the suggestion 

 made in the paper cited, namely, that these small bodies of fine grained 

 gabbro have an intimate connection with the great gabbro intrusions of 

 the region. They are products of the differentiation of the magma, which 

 was the source of these wide spread intrusions, and are alike because de- 

 rived from the common source. The region is regarded as a pctrographic 

 province in which the igneous rocks bear most intimate genetic relation- 

 ships to each other ; and many facts indicate that this is the case. If this 

 supposition be correct, it is to be expected that the fine gabbros may be 

 found anywhere in the vicinity of the large intrusions. 



THIRD VARIETY OF GABBRO. 



Chief Characteristic. — These great intrusions seem to be represented in 

 the region under consideration by a rock of so much interest in its rela- 

 tion to the limestones and gneisses as to merit rather extended considera- 

 tion. It is the third variety of gabbro previously referred to, and, as 

 there stated, is of somewhat unusual character. This fact appears in tlie 

 variable nature of the feldspar, ranging from highly twinned plagioclase 

 to a finely fibrous micro[)erthite, while there is a corresponding change 

 in chemical composition. Thus, different specimens of the rock recpiire, 

 from a strictly petrogra})hic point of view, different names, ranging from 

 gabbro and anorthosite to augite-syenite, but it will for present purposes 

 be considered as a whole under the term gabbro, its geologic relation- 

 ships rather than its pctrographic affinities being the main oljject of in- 

 vestigation. 



Area of Occurrence. — A l)and of this rock stretches across Diana, with a 

 course somewhat north of east, forming tlie soutiiern l)oun<lary of tlic 



XXXVm-Iki.i.. Geol. Hoc. Am.. Vol. fi. 1H'j4. 



