334 E. B. MATHEWS — MARYLAND AND PENNSYLVANIA PIEDMONT 



ment of this micaceous phase of the Setters formation is especially 

 marked along the valley at Stringtown and in the northeastern exten- 

 sion of the limestone valley at Cockeysville. A third instance of this 

 character is found in the small anticline at Warren, especially near the 

 mouth of Royston branch. In all of these instances the micaceous 

 member of the quartzite is seen to underlie the marble and to overlie or 

 to be interbedded with the more quartzose phases of the Setters formation. 

 Moreover, the structure at the various points is such that the occurrence 

 of this garnet mica-schist can not be explained as a faulted-in portion of 

 the Wissahickon mica-schist overlying the marble, as was first supposed 

 by the writer. 



COCKEYSVILLE MARBLE 



The exposures of Cockeysville marble found in the Maryland Pied- 

 mont may be included within an ellipsoidal area with a major diameter 

 of 40 miles extending northeast and southwest from Harford county on 

 the east to Montgomery county on the south and a minor diameter of 

 about 10 miles. The areal distribution suggests that there was in Mary- 

 land a local basin of deposition, but there are few, if any, facts known 

 confirming this suggestion. The maximum development of the Cockeys- 

 ville marble is found in the synclinorium* lying between the anticlines 

 of Baltimore gneiss and quartzite about 10 miles north of Baltimore city. 

 It is here found underlying the Wassahickon mica-schist and overlying 

 the quartzite, the various formations recurring at the surface through 

 numerous foldings, the contact between the marble and the adjacent for- 

 mations lying very close to the present surface of the country. South- 

 west from this larger area of Cockeysville marble the formation may be 

 traced in four exposures along well defined valleys to the vicinity of 

 Clarksville, in Howard county, with but little or no interruption, the for- 

 mation occurring as part of a monoclinal fold to the westward or as the 

 eastern side of the large synclinorium which will be discussed later. The 

 details regarding the southwestern exposure of the Cockeysville marble 

 are not all worked out, and it seems quite probable, from the facts at 

 hand, that there is a fault striking northwest and southeast and extend- 

 ing southeastward to a point near Laurel. The marble occurring in these 

 areas is in the majority of instances rich in magnesium and should be 

 called a dolomite. This is particularly true at the type locality, Cockeys- 

 ville, but there are frequent changes in the amount of magnesium pres- 

 ent, and one often finds magnesium-free or magnesium -poor rocks in 

 proximity to the dolomitic varieties. The changes in composition are 



*The distribution and structure of the Cockeysville marble as exposed in this synclinorium is 

 discussed in detail in the succeeding paper by the author and his associate, Mr W. J. Miller. 



