O. H. Smyth, Jr. — Metamorjphism of a Gabbro. 273 



Art. XXXII. — Metamorphism of a Gabbro occurring in St. 

 Lawrence County, N. T. ; by C. H. Smyth, Jr. 



In the course of a reconnoissance of the crystalline rocks of 

 St. Lawrence County, carried on, during the past summer, for 

 Dr. James Hall, State geologist, the writer examined an area of 

 gabbro which seems of sufficient interest to merit description. 

 The rock affords another indication of the widespread occur- 

 rence of gabbro in the Adirondack region, to which attention 

 has been called in a previous paper,* and also shows striking 

 variations in composition and structure, in part primary, but 

 chiefly secondary. 



The locality is in the town of Russell, about four miles 

 southwest of the village of the same name, and one-half mile 

 east of the well known danburite locality. The prevailing 

 rock of the vicinity is a massive gneiss, generally with a 

 decided red color. This rock probably bounds the gabbro on 

 all sides, although the limits of the latter were found only on 

 the east and west. The extent in this direction is hardly more 

 than one-eighth of a mile. 



It will be convenient to describe, first, the phenomena 

 observed in the Held, and then, to consider the petrographic 

 details of the various phases of the rock. 



The gabbro and associated gneiss form a short steep ridge, 

 with an elevation of fifty to seventy-five feet, just north of the 

 highway. Even in a rapid examination of the region the gabbro 

 could scarcely be overlooked on account of the marked contrast 

 between its coarser portions and the prevailing country rock. 

 These very coarse parts have a decidedly mottled aspect on 

 weathered surfaces, due to imperfect porphyritic structure, 

 with phenocrysts of feldspar ranging up to three inches in 

 length, surrounded by dark green or black material. A more 

 abundant variety has feldspars seldom extending one-half inch 

 in length, and the porphyritic structure is replaced by a more 

 or less perfect ophitic structure. This phase of the rock may 

 become finer, and may assume a nearly granular structure. At 

 the same time there are variations in the relative proportions 

 of feldspar and ferro-magnesian minerals, with corresponding 

 changes of color. These variations in the rock are all primary 

 and may be summarized as a range in grain from very coarse 

 to moderately fine ; in structure from porphyritic to ophitic, 

 and to nearly granular ; in color from nearly black to gray. 

 These changes in character take place very suddenly, and 



* C. H. Smyth. Jr. — On Grabbros in the Southwest Adirondack Region, this 

 Journal, III, xlviii, p. 5-4. 



