MARYLAND GEOLOGICAL SURVEY 135 



The fibrous serpentines exhibit remnants of tremolite and other 

 fibrous amphiboles by means of which their origin can be traced to 

 pyroxenites. 



Serpentine possesses a more uniform appearance under the micro- 

 scope than in the hand specimen. Faintly green, transparent sections, 

 showing in polarized light a confusedly fibrous character and mesh 

 structure, with olivinitic cores and an occasional crystal of pyroxene 

 or tremolite, are a common type of earthy or massive serpentine. 



Chromic iron, magnetite, talc, calcite and quartz are accessory 

 and secondary constituents. 



A preponderance of tremolite, anthophyllite or smaragdite char- 

 acterizes the fibrous serpentines. Tremolite may still show a central 

 area or core of pyroxene. 



Talc is a common secondary product both in the fibrous and massive 

 serpentines. It may completely replace both serpentine and amphi- 

 bole, converting the rock into a soapstone or a steatite-schist. 



The change to talc of the fibrous amphiboles is accompanied by 

 the separation of calcite or dolomite which fills the interstices of the 

 talc. 



A typical example of a calcareous soapstone is found in an exposure 

 on the Octoraro Creek on the southern edge of the State line serpen- 

 tines. It is a light colored bluish green, somewhat soapy rock, but 

 rendered harder and less soapy than it would otherwise be by the 

 presence of a magnesian calcium carbonate. Steatite and dolomite 

 are the constituents. 



The origin of serpentine and soapstone from pyroxenites and peri- 

 dotites is a well recognized fact, and the occurrence of serpentines 

 with such a genesis from localities in Pennsylvania 1 and elsewhere 

 has been described. 



Dike Eocks. 



It has been stated (p. 92) that, beside the ultra-basic dikes in the 

 vicinity of ConowingO', which have been described, there occur in the 

 crystalline formations of the county numerous acid and basic dikes, 

 varying in width from one foot to a mile and a half. 



i Bull. U. S. Geol. Survey, No. 59, Washington, 1890. 



