COCKEYSVILLE MARBLE 



335 



sharp and generally easily recognized by the quarrymen, who are assisted 

 by the fact that the dolomitic marble averages finer grained and richer 

 in magnesium-mica than the better-burning, magnesium-poor rocks. 

 Attempts have been made by acid tests in the field to recognize some 

 stratigraphic distribution of the magnesium and calcium-rich rocks, but 

 these have failed. On the contrary, it has been found that there are rapid 

 sharp alternations of the two types in a way which strongly suggests that 

 whatever dolomitization occurred must have taken place prior to emer- 

 gence from the sea and probably contemporaneously with the formation 

 of the deposit as has recently been described by Professor Branner as 

 occurring on the coast of Brazil. 



No fossils have been found in the marbles, and as they are highly 

 crystalline it is very doubtful if any will be found. The deposit, how- 

 ever, underlies the Wissahickon schist in exactly the same way as the 

 Chester Valley marbles underlie the typical Wissahickon schists, which 

 have been traced continuously into the Maryland areas. 



WISSA HICKON FORMA TION 



The Wissahickon formation in Maryland occurs as a broad band 

 wrapping about the older Baltimore gneiss and limestone areas and 

 occupying practically all the territory between the Blue Ridge and the 

 Coastal Plain deposits except those portions consisting of the sediment- 

 ary rocks already described, the overlapping Triassic sandstones of Car- 

 roll, Frederick, and Montgomery counties and the igneous rocks. This 

 wide belt of rocks may be roughly divided into the Wissahickon schists 

 and the Wissahickon phyllites with included limestone lenses. The line 

 of separation between these two divisions is not marked stratigraphicall}'' 

 by any pronounced change in lithologic character, and it is not entirely 

 clear that the division is a stratigraphic one, although it seems probable 

 that the phyllites in all instances are of as high or higher horizons than 

 the more crystalline Wissahickon schists. It has been clearly shown 

 by recent field work that the more crystalline Wissahickon schists pass 

 by gradations into the less crystalline phyllites and sericite schists. The 

 more crystalline aspects of the Wissahickon formation lie in the eastern 

 part of the Maryland Piedmont, where they are in association with the 

 older, more crystalline sedimentaries and the large masses of intruded 

 gabbros, granites, and other igneous rocks. 



The areal distribution of the Wissahickon formation may be discussed 

 in four parts. The area of Wissahickon schists lying east of the broad 

 phyllite band, which extends southward from the Susquehanna river to 

 southeastern Carroll county, represents the area of highest crystallinity 



