310 G. H. WILLIAMS — STRUCTURE OF THE PIEDMONT PLATEAU. 



Eruptive Mocks. — The rocks of undoubted eruptive origin within the east- 

 ern or more highly crystalline area of Maryland are very abundant and 

 varied. The extensive dynamic metamorphism to which they have been 

 subjected has developed in them many features tending to disguise their 

 original character and to confuse them with highly altered sediments. These 

 rocks have been the most carefully studied and described of any occurring 

 in Maryland, so that for the present purpose it will be sufficient to merely 

 enumerate the more distinct varieties, together with references to the various 

 articles which contain details of their character and alterations. These 

 eruptive rocks may be arranged under three distinct types : * 



Intermediate Type (the most ancient), comprising: 



a. Hypersthene gabbro ;t 



b. Gabbro-diorite and its metamorphic product, hornblende schist; J 



c. Quartz gabbro, Harford county ; 



d. Norite, Harford county ; 



e. Diorite, Ilchester ; § 

 /. Hornbleudite; 



g. Hornblende-biotite-quartz-diorite, Washington. 



Basic Type, comprising : || 



a. Pyroxenite (Websterite) ;^ 



b. Lherzolite ; *'^ 



c. Cortlandtite, Ilchester ; f f 



d. Serpentine, resulting from the alteration of all the preceding basic 



rocks. 

 Acid Type, comprising: 



a. True or binary granite, Guilford ; 



b. Granitite, with allanite-epidote growths ; 



c. Hornblende granite, Garrett Park ;§§ 



d. Granite porphyry, EUicott City ; 



e. Augen-granite gneiss, Texas, Baltimore county ; 

 /. Felsite (quartz-porphyry), in dikes at Relay ; 



g. Pegmatite (muscovite-biotite). |||| 



Rocks whose eruptive origin is either undoubted or most probable cover 

 at least half of the now exposed surface within the eastern or more crystal- 



*See Am. Geologist, vol. "6, Julv, 1890, p. 36. 



t Bull. U. S. Geol. Survey, no. 28, 1886, p. 18. 



tlbi.1., p. 27. 



g Hobbs: Johns Hopkins University Circulars, no. 65, 1888; and Trans, Wis. Acad. Sci., vol. 8, 

 November 10, 1890, p. U7. 



II Bull. U. S. Geol. Survey, no. 28, 1886, p. 50. 



i[ Am. Geologist, vol. 6, 1890, p. 40. 



** Ibid., p. 38. 



ft Hobbs : Loc. cit. 



jj Hobbs: Johns Hopkins University Circulars, no. 65; TschermaU's min. petr. Mitth., vol. 11, 

 1889, p. 1. 



gg Keyes : This volume, p. 321. 



nil Johns Hopkins University Circulars, no. 38, vol. 4, 1885. 



