90 THE CRYSTALLINE ROCES OF CECIL COUNTY 



The soil formed from the mica-gneiss contains sand, clay, muscovite 

 and biotite. It has a yellow color and sparkles with muscovite. 



The conspicuous presence of this mineral has given rise among 

 the farmers to the name " isinglass soil." 



In this and other characters it presents a considerable contrast to 

 the soil derived from the ultrabasic and basic rocks bordering the 

 mica-gneiss in the western half of the county, but graduates almost 

 imperceptibly into the less micaceous clay loam derived from the 

 granite-gneiss adjacent to the mica-gneiss in the eastern half of the 

 county. 



IGNEOUS INTRUSIVES. 



The mica-gneiss constitutes the only sedimentary formation among 

 the crystallines of Cecil county. 



That the other members of the crystalline belt are igneous material, 

 more or less metamorphosed, will become clear as the discussion 

 proceeds. 



These igneous rocks follow closely the strike of the sedimentary 

 formation. They are successively exposed on the Susquehanna river 

 and trend northeast, passing out of the county into Pennsylvania and 

 Delaware. 



The Granite-Gneiss. — Granitoid rocks are exposed on the Sus- 

 quehanna for some nine miles, but not uninterruptedly. An inclu- 

 sion of mica-gneiss separates the granite-gneiss into two portions. 

 The southern portion is traversed by dikes and larger intrusive 

 bodies. The Susquehanna granites are medium-grained, light-colored 

 rocks, irregularly marked by dark biotite or hornblende. 



These granitoid rocks, so finely exposed by the erosion of the Sus- 

 quehanna, and so favorably situated for quarrying, have long been 

 recognized as the most satisfactory building stone of the county. 



Their extension northeast from the Susquehanna across the county 

 has not previously been recognized. 



Like the other formations, the granite-gneiss crosses the county 

 to the northeast and disappears under Potomac gravels in the vicinity 

 of Newark, Delaware. 



