274 0. H. Smyth, Jr. — Metamorjphism of a Gabbro. 



the different phases are most irregularly distributed, seem- 

 ing to conform to no law. The prevailing variety is that hav- 

 ing the coarse ophitic structure. In spite of this structure, the 

 rock is classed with the gabbros, rather than with the diabases, 

 on account of its coarseness, the character of its component 

 minerals, and its affinity with other rocks of the region which 

 are undoubted gabbros. 



The primary variations in the rock suffice to give considera- 

 ble diversity to different portions, within a limited area, but 

 this diversity is greatly intensified by certain secondary modifi- 

 cations of structure and composition. As a result of the com- 

 bined effect of primary and secondary variations, it would be 

 easy to collect, within an area of a few square rods, a half dozen 

 or more specimens whose appearance, even in thin section, 

 would scarcely suggest that they had any connection with one 

 another. 



The secondary changes are of two kinds. The first might 

 be classed as degradational in nature, the mineral constituents 

 decomposing, with the formation of reddish masses, blotched 

 with green. These masses are scattered through the unchanged 

 rock very abundantly, and, at first sight, look like inclusions of 

 red gneiss, but on closer inspection they show a less crystalline 

 aspect than the gneiss, being rather inclined to an earthy sur- 

 face, and they, further, pass by gradual transitions into the 

 gabbro. 



The second modification is mainly structural, so far as the 

 megascopic aspect is concerned, but is accompanied by certain 

 peculiar transformations in the minerals of the rock. In this 

 instance the gafcbro loses its massive character and passes over 

 into a well developed gneiss, sometimes almost a schist. As 

 the normal structure gives place to a parallel arrangement the 

 constituent minerals assume a more or less rounded contour, 

 and the resultant gneiss is, in consequence, decidedly granu- 

 litic. The passage from coarse, massive gabbro into fine, 

 granulitic gneiss, and back into gabbro, is repeated over and 

 over again, and takes place within a few feet. Specimens of 

 the extreme types differ greatly in appearance from each other, 

 and, in the absence of the intermediate phases, would scarcely 

 be suspected of having a common origin. 



At several points this gneissoid gabbro shows still another 

 modification of a somewhat peculiar character. This consists 

 principally in the development of platey masses of hornblende, 

 which cut across the foliation of the gneiss at a high angle, 

 approximating, as a rule, a right angle. These plates of horn- 

 blende range in thickness up to about half an inch, and extend, 

 interruptedly, for several feet in a horizontal direction. There 

 are no sections to show their vertical extent, but it doubtless 



