134 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



Three-eighths of a mile southeast of Conowingo Station, on the 

 Susquehanna river, there are three pyroxenite dikes and four that 

 originally were peridotite. The latter are more or less completely 

 altered to soapstone. They vary in width from twenty to forty feet 

 approximately and strike northeast. The peridotites are more uni- 

 formly coarse-grained than the material which has thus far been 

 discussed. They are dark brownish green to black in color, and 

 sometimes exhibit the characteristic lustre-mottling, due to the inter- 

 ruption of a lustrous cleavage surface of pyroxene by dull grains of 

 included olivine. 



The constituents are hypersthene, diallage and olivine. Feldspar 

 is an occasional accessory. Serpentine, tremolite, steatite and mag- 

 netite are secondary products. 



Olivine is the oldest constituent and occurs as included crystals 

 in the pyroxenes, or is the predominating mineral. It is always more 

 or less altered to serpentine with the liberation of iron oxides. The 

 pyroxenes are less likely to be altered, but tremolite, serpentine and 

 steatite have in some cases formed at the expense of the pyroxene. 

 As in the case of olivine-free rocks, either the orthorhombic or mono- 

 clinic pyroxene may predominate, thus giving rise to two varieties 

 of peridotite, the hyp ersthene-oli vine rock or harzbergite, and the 

 diallage-olivine rock or wehrlite. 



The association of the peridotites and pyroxenites with the ser- 

 pentines and soapstones, and the more or less complete serpentini- 

 zation of well recognized peridotites and pyroxenites, are presump- 

 tive evidences that both types were once far more extended but are 

 now represented by the serpentine. 



SERPENTINE. 



The great range in color and texture of the serpentine and asso- 

 ciated material has already been presented. Every variety and shade 

 of green characterizes them. The texture may be earthy, fibrous or 

 massive. 



Massive serpentines more generally disclose a peridotitic origin, 

 but even the earthy serpentines show remnants of original olivine. 



