MARYLAND GEOLOGICAL SURVEY 111 



On the south the succession is more or less concealed by overlying 

 deposits. That a similar succession might be uncovered here, is 

 indicated by the reappearance of hornblende in the southern margin 

 of the granite-gneiss and by the occurrence of outliers, in the gravel 

 areas to the south, of gabbro, pyroxenite and serpentine. These out- 

 liers occur in the neighborhood of Elktou on the margin and south 

 of the plateau. They are part of a more extended gabbro-pyroxenite 

 area in Delaware. 



In Harford county, southwest of Havre de Grace, a continuation 

 of this gabbro belt has been mapped. 



The existence of a zone of intermediate and connecting types 

 between granite and gabbro, as suggested by Dr. Leonard, 1 may easily 

 lead to an unwarranted emphasis of insignificant connecting types; 

 but, in general, the writer agrees with his observations rather than 

 with those of Dr. Grimsley, 2 who describes an eruptive contact be- 

 tween granite and gabbro on Octoraro Creek in the vicinity of 

 Porter Bridge. 



The granite contains here, as elsewhere, dark, fine-grained basic 

 segregations. They do not change in character or increase in number 

 as the gabbro belt is approached. The rocks of this belt contain 

 similar segregations. 



These oval or irregularly shaped patches vary in size from one 

 inch (26 millimeters) to two or three feet (60 to 90 centimeters), 

 and are frequently foliated in character owing to an excess of biotite 

 over the other constituents, quartz and feldspar. Their longer direc- 

 tion and their foliation, when present, are parallel to the schist- 

 osity of the enclosing rock, which is usually coarser grained. 

 There has been no difference of opinion as to the character of these 

 segregation patches. The following table of their silica percentage 

 and specific gravity determination with that of similar material form 

 elsewhere, is taken, with some necessary corrections, from Dr. Grims- 

 ley's paper: 



lAmer. Geol., vol. xxviii, 1901, pp. 167-168. 



2 Jour. Cincinnati Soc. Nat. Hist., vol. xvii, p. 65. 



