10 . UNIVEKSITY OF VIRGINIA PUBLICATIONS 



mineral in the Maryland granites, and has been noted in some of the 

 granites of the Carolinas. Chlorite, epidote, and a light-colored mica, mus- 

 covite, are the common secondary minerals. 



VARIETIES. 



Based on mineral composition the biotite type of aeid-irruptive rock is 

 the predominant one, with the muscovite-bearing biotite type quite common. 

 Miiscovitic and hornblendic types are rare, and epidotic ones are noted in 

 certain localities in Maryland, Virginia, and North Carolina. Structurally, 

 there are three rather well marked phases, namely, granitoid (even-granular 

 granites), porphyritic (porphyritic granites), and schistose (granite- 

 gneisses). The even-granular and porphyritic textures usually grade into 

 each other and are regarded as textural variations of the same rock-mass. 

 The granite-gneisses are derived from original massive granites by meta- 

 morphism, and, as a rule, do not grade into the massive granites, but are 

 sharply defined from them. Even-granular and porphyritic textures likewise 

 characterize the schistose type (granite-gneisses). 



INTERMEDIATE CHARACTER OF THE ROCKS. 



Although these rocks have been described as granites, an examination 

 of the results obtained from their investigation strongly impresses one that 

 they are not normal or true granites, but occupy an intermediate position 

 between granite and quartz-diorite.^ They differ essentially in mineral com- 

 position from the intermediate type of rock of fairly constant character so 

 abundant over parts of the Pacific Coast region of the United States, 

 described and mapped by the geologists of the United States Geological 

 Survey as granodiorite, in the practical absence of hornblende, the composi- 

 tion and ratio of feldspar content, and the frequent presence of muscovite 

 associated with the biotite. 



'The rhyolite-granite and the trachyte-syenite groups of rooks are characterized 

 by the predominance of alkali feldspars, commonly orthoolase. On the other hand, 

 the andesite-diorite group is characterized by dominant sodic-lime (plagioclase) 

 feldspars. Between these groups of rocks there occur all gradations, with respect to 

 the ratio of potassic and sodic-lime feldspars. Considerations of convenience, there- 

 tore, based on the ratio (amounts) of orthoelase and plagioclase of the rocks, have 

 led to the formation of an intermediate group, whose granitoid members are desig- 

 nated as monzonites. (Brogger, W. C. Die Eruptivgesteine des Kristianiagebietes, 

 1895, II, 21). Quartz monzonite is intermediate between granite and quartz diorite, 

 and monzonite between syenite and diorite. These intermediate rocks contain 

 potassic and sodic-lime feldspars in approximately equal amounts, with or without 

 quartz, and with subordinate amounts of the ferromagnesian minerals. (Clarke, F. 

 W. The Data of Geochemistry, Bull. 330, U. S. Geol. Survey, 1908, p. 380). 



