MARYLAND GEOLOGICAL SURVEY 89 



and five-eighth miles, and a length of from seven to ten and a half 

 miles. 



As the separation between it and the formation to the south must 

 be made almost wholly on the character of the soil, and is a separation 

 between soils derived from the decay of a muscovite-biotite-gneiss and 

 the decay of a biotite-granite-gneiss, sometimes carrying muscovite, 

 this boundary is somewhat hypothetical. 



In a general southwest direction from Calvert there are a few scat- 

 tered occurrences of a micaceous (muscovite^biotite) gneiss. These 

 may be either alteration phases of the granite-gneiss under pressure, 

 or inclusion of the mica-gneiss. 



There is a considerable area of rock to the southwest outcropping 

 on the Susquehanna, where it is about a mile in width, which extends 

 eastward, pinching out about a half mile east of Liberty Grove. 

 This exhibits the characteristics of the mica-gneiss and is inter- 

 preted as an inclusion of the mica-gneiss within the igneous forma- 

 tions. 



The mica-gneiss offers little resistance to weathering and is thor- 

 oughly decayed when exposed in road cuts and railway cuts. The 

 Susquehanna and the Big Elk Creek alone cut into the solid rock. 

 The formation is so varied in character and in the amount of mica or 

 of feldspar carried by it that, while often quite worthless for 

 economic uses because of an excess of mica, it may locally become a 

 fair building-stone. 



It has been used as such in Pennsylvania, but is not quarried in 

 Cecil county. 



It is abundantly traversed by pegmatite and quartz veins; the 

 former two feet and upwards in width, the latter varying from a 

 fraction of an inch to fifty or more feet. These veins usually strike 

 parallel to either the cleavage or joint planes, and are composed 

 of quartz only or of quartz, an alkali feldspar, muscovite and biotite. 



In Pennsylvania the pegmatites assume a commercial importance. 

 At many localities they are quarried for feldspar or for the kaolin 

 to which the feldspar has altered. The " spar " quarries of Cecil 

 county are not in the mica-gneiss, but traverse the igneous formations. 



