io6 



Cincinnati Society of Natural History. 



Rutilc was not observed in the schist, but it occurs in the 

 staurolite in short crystals in considerable amount. 



Chlorite is more abundant in the schist than in the stauro- 

 lite, and while much has originated from the decomposition 

 of the garnets, more has probably come from the bleaching 

 of the biotite. The presence of a strongly pleochroic biotite 

 lends weight to this opinion. 



In the quartz of the staurolite occur small elongated color- 

 less crystals which were thought to be apatite, and the presence 

 of phosphoric acid, as shown by the analysis, adds weight to 

 this opinion. 



II. — ISOLATION OF THE RARER CONSTITUENTS OF THE 



CECIL COUNTY ROCKS. 



In the second chapter of this paper, it was noted that the 

 granite exposures, along the road sides and in some of the 

 creek banks, are so much weathered as to conceal the hard 

 rock with a stratum of soil of varying depth. It is impossible 

 to study these granites in thin sections. One is able, however, 

 in such cases, to separate out of this earth a fine residue con- 

 taining the rarer and heavier minerals of the granite. This 

 may be accomplished by a separation through specific gravity 

 in water, a process which is valuable in concentrating minerals 

 that are sparingly distributed in the solid granite. 



Thurach* first recognized the value of such separations, 

 and by this means he made a study of the rarer constituents 

 of many Central European rocks. He washed the powdered 

 rock with water in porcelain dishes, allowing the lighter 

 minerals to pass over the edge of the dish with the liquid, 

 which was thrown out by a quick motion of the hand. His 

 study of residues thus obtained added much to our knowledge 

 of the heavier rock constituents. A more convenient appa- 

 ratus was employed by Derby t in Brazil, and the mode of 

 procedure, though used without any knowledge of Thlirach's 

 work, is almost the same. 



In the study of these decomposed granites of Cecil County, 

 I have used the apparatus suggested by Professor Derby, 



-Zirkone und Titan Mineralien in den Gesteinen (1884). 



fDescribed very fully in Proceedings Rochester Academy of Science. Vol. I, p. 



198, (1891). 



