ORBICULAR GABBRO-DIORITE 



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granite. (2) Two textural fades of the rock having approximately 

 the same mineral composition, are observed, namely, (a) orbicular or 

 nodular, and {h) granitic. (3) In the nodular facies the nodules com- 

 pose vastly the greater bulk of the rock. They are of varying, but mod- 

 erately large, size, usually crowded close together, with a small amount 

 of matrix or interstitial filling, largely composed of feldspar and very 

 subordinate amount of the other minerals. (4) The nodules are uni- 

 formly dark in color, very basic in composition, and, with but few 

 exceptions, are made up almost entirely of the dark silicates, diallage, . 

 and hornblende. (5) The structure of the nodules is radial and not, 

 as frequently observed in such rocks, concentric, or both. (6) Micro- 

 scopic evidence suggests that the nodular structure is a product of 

 magmatic differentiation. 



Thomas Leonard Watson. 



Geological Laboratory of Denison University, 

 Granville, Ohio. 



