MARYLAND GEOLOGICAL SURVEY 93 



meta-gabbro. This belt bas an exposure for a mile and a balf on the 

 Susquehanna river and becomes narrower to the northeast, disappear- 

 ing altogether about three-fourths of a mile east of Lombard. 



Outside of this belt, gabbroitic rocks occur about one mile south 

 of Iron Hill station, on Grays Hill and on the right bank of Big Elk 

 Creek, two miles north of Elkton. These are outliers of the gabbro 

 area of Chestnut Hill and Iron Hills in Delaware. 



The gabbro is typically a medium-course-grained granular rock, 

 varying in color from a greenish grey to a brownish black. The 

 medium dark tints are prevalent and serve to separate the gabbro 

 from the granite-gneiss. The constituents can usually be distin- 

 guished in the hand specimen and are a reddish brown or greenish 

 brown hypersthene, often with a bronzy lustre, green diallage and a 

 greenish grey or opaque white feldspar. 



To the southward these fresh appearing hypersthene-gabbros or 

 norites grade into rocks in which the hypersthene is altering or has 

 completely altered to hornblende. Such a zone of hornblende rocks 

 or meta-gabbros borders the fresh norite on the south and composes 

 the mass north and east of Calvert. These rocks, in turn, show the 

 addition of quartz, a blue quartz conspicuous in the hand specimen, 

 and with a slight increase in the acidity of their feldspar become 

 quartz-biotite-hornblende-gabbro. This quartz-gabbro passes almost 

 imperceptibly into hornblende-biotite-granite and this into a biotite- 

 granite, the formations thus increase in acidity southward. 



Meta-Pyroxenites and Meta-Peridotites (Amphibolite, Serpen- 

 tine and Soapstone). — For five-eighths of a mile from the northern 

 limit of the gabbroitic material to Bald Friar, the Susquehanna cuts 

 through a belt of soft and sometimes soapy greenstones. 



These greenstones represent various phases in the metamorphism 

 of non-feldspathic igneous rocks. This belt widens to the northward 

 and, with a width on the Mason and Dixon Line of three and one- 

 eighth miles, passes, north of Rock Springs, into Pennsylvania, 

 where it trends to the east and sweeps southward into Cecil county 

 at four points between Pock Springs and Pair View. The last 

 exposure is some sixteen and a half miles east of the Susquehanna 



