396 JULIEN 



signify a pitch of the axes of the folds toward the northeast, 

 directly opposite to the usual pitch in this region/ 



All the features intimate intrusion of the original gabbro 

 before the folding of the stratum of gneisses, and effacement of 

 the ascending dikes as a natural result of deformation and 

 occlusion. 



Petrograpliic Description. — The rock of the belt consists 

 mainly of thinly laminated quartz-diorite schist ordioritic gneiss, 

 similar to the hornblendic gneisses and schists of the island. It 

 differs however in the general distribution throughout the fine- 

 grained hornblendic groundmass of dull black grains and flakes 

 of hornblende, 0.5 to 3.0 centimeters across, and about i to 2 

 centimeters apart. These mottle its fresh surfaces with black, 

 shining spots and project over the weathered crust in dark 

 lumps, as commonly observed on the weathering of diorites 

 and gabbros. They seem to represent ancient phenocrysts — 

 originally, it may be, of pyroxene — now altered to hornblende, 

 comprising about a third of the hornblende of the rock, and 

 with forms distorted by shearing. Their outlines are generally 

 irregular, rectangular, rhombic, and very often rounded or ovate, 

 producing an augen-structure in miniature. All the indications 

 are of an original rock of gabbroitic habit, certainly without any 

 resemblance to diabase. 



The more gently inclined beds at the top of one fold show a 

 coarse banding, made up of light and dark layers, 3 to 50 centi- 

 meters in thickness, which suggest bands of segregation or flow. 

 Some of these are accentuated by impregnation with white 

 pegmatite in seams or in ovate nodules, less than a decimeter in 

 length, arranged in parallel planes ; or these may consist of milky 

 quartz or of pure feldspar, often a white plagioclase. Every- 

 where, however, throughout this bed, there is entire absence of 

 the minor crumpling and corrugation of laminae of common 

 occurrence elsewhere on the island. 



In the general groundmass around the large black grains 

 hornblende predominates, mostly in slender blades or flattened 

 prisms, jet-black and shining, 2 to 5 centimeters long. These 

 are separated by grayish white parallel films, 0.5 to i.o milli- 



1 J. D. Dana, Am. Jour. Sci., (3), XX, 1880, 361. 



