460 Hawkins — Notes on the Geology of Rhode Island. 



Characteristics of this Gabbro Group. 



The gabbros when fresh are typical rocks of medium 

 size of grain; with increase of hornblende these pass 

 into a black rock with bladed structure, usually rather 

 coarse. Under the hand lens the coarser gabbros show 

 laths of white plagioclase (also sometimes dark-colored), 

 hornblende and pyroxene crystals, frequently some fresh 

 bio tit e plates, and black grains of magnetite and ilmenite. 

 The microscopic characters of the group are as follows : — 



(1). Diabasic texture with labradorite. 



(2). Graphic intergrowth of magnetite and ilmenite with 

 biotite and basaltic hornblende. 



(3). Corona structure involving olivine and iron ores with 

 rims of hornblende, biotite, and hydrous alteration products. 



(4). Inclusions of quartz carrying rutile and constantly 

 moving bubbles. 



The chemical characters of the group are generally 

 as follows : — 



(1). Low in Si0 2 , MgO, Na.O, K 2 0, P 2 5 , H 9 0. 

 (2). High in Feb and CaO. 



Endomorphic and Exomorphic Chemical Changes Due to 

 Contact Metamorphism. 



(Production of Hybrid Granites, Pseudo-Diorites, and 

 Limestones.) 



The characteristics of many of the contacts of granite 

 with the basic rocks have been mentioned above. In 

 other instances, however, a well-defined gabbro, of mas- 

 sive, noncrystalline nature, may be surrounded for a dis- 

 tance of a mile or more by a complex of hybrid rocks 

 resulting from the intimate intermixture of gabbroid 

 material with granite, evidently a partial assimilation of 

 the basic material by the latter rock. (Compare Emer- 

 son, idem, 173-175.) The injection gneisses of this type 

 are light gray, or streaked with wavy light bands of 

 granite or material resulting from pneumatolytic effects 

 from the latter. Of this sort of occurrence the finest 

 example is found at South Foster, of which further men- 

 tion will be made. (Compare Dana Diorite of Emerson, 

 especially figs., 245 (idem).) 



