GEOLOGY OF FRANKLIN COUNTY 111 



As far as can be judged from the less metamorphosed portions, 

 these gabbros show a similarity in mineral content and in struc- 

 ture which is unusual in gabbroic rocks. The essential original 

 minerals are in every case, a plagioclase feldspar (usually labra- 

 dorite), augite and magnetite (always titaniferous). In some of 

 the rocks hypersthene must be added to this list, but in these it 

 seldom equals the augite in amount, so that norite, an intrusive 

 rock constituted essentially of labradorite feldspar and hypers- 

 thene, is rare in Franklin county. In all cases in which the orig- 

 inal structure has been preserved, it is found to be ophitic, that 

 is the feldspar is in long, lath-shaped crystals separated by, and 

 partially embedded in, the large, stout prisms of the augite. 

 These primary feldspars and augites invariably hold a multitude 

 of minute, microscopic inclusions, the augite specially containing 

 them in such numbers that in thin section it would often be im- 

 possible to make out the color of the mineral, were it not for the 

 fact that a narrow outer zone is comparatively free from them. 

 These inclusions in the augite are mainly opaque under high 

 powers and are probably of magnetite or ilmenite. The feldspar 

 inclusions are for the most part small augites arranged linearly 

 parallel to the long axis of the laths. 



From the extinction angles shown by the feldspars in the 

 various slides it is quite certain that they show a range from 

 andesin to anorthite in the different specimens, with labradorite 

 the more usual one. The augite is of an exceedingly pale gray 

 green shade, nearly colorless in very thin sections. 



In addition to the foregoing, even the least metamorphosed 

 rocks show much rather finely granular material, either in clumps 

 or in trains, consisting mainly of garnet, augite, hypersthene or 

 bronzite, labradorite and hornblende, to which biotite is some- 

 times added. These are all in equi-dimensional grains, and were 

 forming at the same time, as none of them show crystal bound- 

 aries. In the more fully metamorphosed rock this granular ma- 

 terial increases till it comes to constitute the whole, all traces of 

 the original minerals having disappeared. In some slides the 

 large, original augites can be seen trailing out into this granular 



