94 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



river. The two more easterly of these occurrences are disconnected 

 at the surface from the main mass by gabbro. 



There are also five lens-shaped inclusions of these ultra-basic 

 intrusives within the gabbro area, three within the mica-gneiss for- 

 mation, and some dikes of the same, south of Conowingo and at 

 Oakwood. The dikes will be discussed later (pp. 97-100). 



The greater portion of this ultrabasic material has undergone com- 

 plete alteration to serpentine. So thorough is the transformation 

 throughout the mass of the formation that the original characters 

 of the rock may only be determined through microscopic study. 



At scattered localities along the southern periphery of the serpen- 

 tines, gradations into only partially altered pyroxenites and peridotites 

 may be observed in the field. Such passage into well-recognized 

 pyroxenites and peridotites is also exhibited in the dikes above men- 

 tioned and in the lens-shaped included masses. 



This is the belt that has been generally known as the " State line 

 serpentines " because the serpentine occurs for fully sixteen miles 

 along the boundary between the states of Pennsylvania and Maryland. 

 The serpentine is most varied in color and general appearance. It 

 ranges in tint from a light buff, or a light yellowish green, to a rich, 

 deep emerald green or a dark blue green. 



Where the serpentine possesses a soft earthy texture, the colors are 

 usually light in tint. Where it is compact and massive there is a 

 deepening of the green tones, while a schistose or fibrous character 

 is associated with a greyish green shade, which is due to the develop- 

 ment of either talc, asbestos or tremolite. 



The rock possesses no original structural planes, but exhibits more 

 than one system of secondary planes produced by pressure acting 

 upon a massive rock. The master-joints are nearly vertical and strike 

 northeast and southwest. The joint faces are often slicken-sided. 



The metamorphism of the original ferromagnesian silicates (pyrox- 

 ene, amphibole and olivine) into the hydrous magnesian silicate, 

 serpentine, is accompanied by the liberation of iron oxides and silica, 

 in the form of hematite or limonite and quartz or opal, or a yellow 

 limonitic jasper or chalcedony. 



