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U2 C. R. KEYES — MARYLAND GRANITES AND THEIR ORIGIN. 



border. The constituents have undergone much crushing and the feld- 

 spars arc scarcely recognizable. The biotite is nearly all bleached, and 

 chlorite is very abundant. There is also presenl secondary epidote and 

 muscovite, and a few larger decomposing cubes of pyrite are scattered 



through the mass. 



The margin of the gneiss blocks is dark colored and much finer in 

 grain. No traces of pressure are noticeable, and apparently it has been 

 completely recrystallized. Biotite is very abundanl in small Hake-. 



oriented in the direction of foliation. A little plagioclase and orthoclase 

 occur, and also small quantities of pyrite. 



Proofs of their eruptive Nature. — The proofs thatthe granites in question 



are eruptive in nature is deduced from several different and independent 

 sources : 



a. From field relations. 



6. From inclusions. 



c. From contact phenomena. 



d. From microscopic examinations. 



a. Their field Relations. — As stated elsewhere, the eastern half of the 

 Piedmont region consists chiefly of gneisses broken through in numerous 

 places by undoubted eruptives, such as gahbro, diorite and pyroxenite, 

 until these rocks occupy fully one-half of the present exposed surface 

 of the district. 



Now, a careful tracing of the granites shows that they have cut in- 

 discriminately across the igneous rocks mentioned as well as the gneiss, 

 passing uninterruptedly from one petrographically distinct mass to 

 another. In other words, the acidic types of crystallines to all appear- 

 ances seem to he younger in age than the gabbros and the most basic 

 rocks, as if they too had broken through all the other eruptives. Near 

 some of the granitic masses, true granitic and felsitic dikes are clearly 

 defined, which would ordinarily he regarded a- apophyses of the main 

 body were the rock regarded as an eruptive. Furthermore, at Dorseys 

 Run station, for instance, large exposures show the granite forced 

 between and spreading widely apart enormous layers of twisted and 

 puckered gneiss: while at Woodstock lucre blocks of gneiss are com- 

 pletely inclosed in the granite. 



As already remarked, the line of contact between the granite and the 

 contiguous rock is seldom determinable exactly, on account of profound 

 superficial decay. Vet occasionally artificial excavations into the acid 

 rock reveal.- clearly such contacts. A good example is that found at 

 the new quarry opened about two mile- northwest of Garrett park, 



