ROCKS OF THE REGION 351 



Feet Inches 



Coarse grained, pure, calcitic 10 



Fine grained, micaceous, dolomitic 2 



Faulted, beds disturbed, calcite veins 10 



Fine grained, dolomitic beds 12 



Medium grained, light blue, calcitic 4 



Total 73 



The different layers within the formation vary widely in coarseness of 

 grain from the fine, almost statuary, marble obtained at the Beaver Dam 

 quarries at Cockeysville to the coarse so-called alum stone, in which the 

 invividual grains may reach a diameter of one-half, three-fourths, or even 

 If inches, as is found in many parts of the region. So far as any general- 

 izations can be drawn regarding the variations of texture, it appears that 

 the dolomitic or magnesian-rich varieties are finer grained and more com- 

 pact as compared with the purer lime carbonate rocks,'which are generally 

 coarsely crystalline. Another feature of these rocks is the presence of im- 

 purities along fairly well defined lines which have for the most part been 

 recrystallized into magnesium silicates. These lines of impurity may sep- 

 arate the different lime-rich beds from those rich in magnesium, or, what 

 is more commonly the case, dolomitic layers from each other. It seems to 

 be rather generally the rule that the impurities are more intimately asso- 

 ciated with the layers rich in magnesium than with those rich in lime, 

 but frequent exceptions may be found to this statement. Among the 

 most common accessory minerals found in this formation are phlogopite, 

 biotite, muscovite, and iron pyrites ; tremolite, quartz, and occasional 

 tourmaline are, however, more frequently found within the limits of in- 

 dividual beds. 



The general rule employed in field work, based upon innumerable acid 

 tests, has been that the coarser grained beds are calcitic and the fine 

 grained beds are dolomitic ; each type is white, cream, brown, or some- 

 what dirty in color. 



WISSAHICKON SCHIST 



This formation consists of a series of highly micaceous, very schistose, 

 and often crinkled aggregates of quartz, more or less chloritized biotite 

 and garnet, with accessory orthoclase, cyanite, staurolite, etcetera. With 

 the increase of feldspar the rock passes into a gneiss. This, however, is 

 less distinctly banded than the gneisses of the Baltimore formation. The 

 individual beds in this type are only indistinctly marked, and their sepa- 

 ration from the well-defined lines of foliation is often attended with con- 

 siderable difficulty. The dip of the foliation varies somewhat, but is 

 usually about 45 degrees, while the effective dip of the individual beds, 



