OCCLUSION OF IGNEOUS ROCK 431 



of the latter from intricate folding and intense compression. 

 Although Williams remarked on the scarcity of quartz in the 

 first two of the above types, quartz-bearing gabbro has since 

 been found ^ at several points near Baltimore, the quartz occupy- 

 ing from lo to 33 per cent, of the whole rock, and in Cecil 

 county even quartz-diorites and tonalites have been recognized. 

 A recent examination of the igneous rocks of that county^ has 

 confirmed the evidence of magmatic differentiation, resulting in 

 a central body of intermediate activity, penetrated by intruded 

 material both of greater acidity and greater basicity, in the 

 following succession : Basic granite (biotite- or hornblende- 

 quartz-monzonite), sometimes altered to gneiss, containing dikes 

 more acid in character, altered to meta-rhyolite ; quartz-biotite- 

 hornblende-gabbro ;^ quartz-hornblende-gabbro, altered also to 

 meta-gabbro and meta-quartz-gabbro ; hornblende-norite and 

 quartz-norite ; norite and hypersthene-gabbro ; pyroxenite and 

 peridotite, altered also to amphibolite, serpentine and soapstone. 

 With the serpentine amphibole schists are associated, containing 

 asbestus. tremolite, anthophyllite, actinolite or chlorite, and rep- 

 resent the metamorphism of pyroxenites. The granite is crossed 

 by throngs of gabbroitic dikes, a reversal of the relationship 

 established elsewhere in the belt and on Manhattan Island. A 

 prominent center of this ancient eruptive activity in Maryland 

 lies close to the site of the city of Baltimore, which stands on 

 the edge of the crystalline tract, where it is crossed by the Pa- 

 tapsco river. 



Deiazvare. — In northern Delaware a series of crystalline 

 schists, known to be of Cambro-Silurian age, offers "the most 

 complicated and striking examples of contortion." ^ These 

 comprise micaceous schists which lie above the Trenton and 

 possibly above the Hudson river schists ; a highly crystalline 

 magnesian marble (Calciferous limestone) ; and a coarse quartz- 



' U. S. Grant, Johns Hopk, Univ. Circ. No. 103, 1893. 



2 Bascom, loc. cit., 83-I48. 



3 The analysis by Hillebrand (Bascom, loc. cit., 124) approximates that of the 

 quartz-biotite hornblende schist of Manhattan Island (Julien, loc. cit., 439). 



«F. D. Chester, Froc. Acod. Nat. Sci. Phila., XXXVI, 1884, 237-249. 



