142 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



^To analysis was secured of the Cecil county diabase, which undoubt- 

 edly is uniform in composition with that of the Pennsylvania diabase. 



Several analyses have been made of the Triassic diabase of 

 Pennsylvania, and the two which have been selected represent mater- 

 ial not far from the State line. 



They show the normal composition of a labradorite-augite rock. 



Summary. 



The crystallines of Cecil county embrace metamorphic rocks of 

 aqueous origin and of igneous origin. The former class is represented 

 by a mica-gneiss of a heterogeneous lithologic character with stratifi- 

 cation planes dipping southeast. The thickness of the formation 

 does not admit of determination. The structure is presumably one 

 of gentle overturned folding, giving a dip varying from a low angle 

 southeast to verticality. The relation of this formation in Penn- 

 sylvania to known Paleozoics, and to presumably Paleozoic sedi- 

 ments, appears to be that of conformable stratification. 



The possibility of a faulted structure is also recognized. The 

 presence of a fault renders the formation pre-Paleozoic. In its 

 absence it is presumably Paleozoic. 



Into this mica-gneiss have been intruded: 



1) igneous material of medium acidity (quartz-monzonite), which 

 in turn includes intrusions of greater acidity (rhyolite) and of greater 

 basicity (gabbro); 



2) basic igneous material (norite and quartz-norite) and 



3) ultra-basic material (pyroxenite and peridotite). 



All of these types show metamorphism. The quartz-monzonite is 

 altered to a gneiss, the rhyolite to a meta-rhyolite, the gabbro, norite 

 and quartz-norite to meta-gabbro and meta-quartz-gabbro, the pyrox- 

 enite and peridotite to amphibolite, serpentine and soapstone. In 

 the acid and basic rocks metasomatism has been of the nature of 

 epidotization, zoisitization and uralitization. In the ultra-basic rocks 

 uralitization, serpentinitization and steatitization have been the pro- 

 cesses of metasomatism. 



The age of these intrusives must remain problematical until the 

 age of the mica-gneiss, into which they intrude, has been determined. 



