152 Mathews — Structure of the Piedmont Plateau. 



un proven, but, as already outlined in the first part of this paper, 

 the sequence here is comparable to that of the similarly situated 

 areas farther north where they have been correlated with less 

 metamorphosed beds bearing fossils. The litliological features 

 and structural characteristics are the same in the different 

 regions and it is probable that this correlation is the true one 

 and that it must, perhaps forever, remain unproven for certain 

 portions of the Maryland region where the beds are separated 

 from other sedimentaries by igneous rocks. As the work now 

 in progress under the joint auspices of the Maryland and U. S. 

 Geological Survey goes forward, it may be possible to correlate 

 on structural grounds the limestones with fossil-bearing beds 

 near Frederick, and it is possible that well-preserved fossils may 

 be found in the less metamorphosed sedimentaries of the Fred- 

 erick valley to the west of Parr's Ridge, as Mr. Keith has 

 already detected fragments in this i*egion. 



Igneous Rocks. — Through these sedimentary rocks have been 

 intruded large masses of igneous material which have consoli- 

 dated into granites, gabbros, and other igneous types. For- 

 Imerly it was regarded that the masses were intruded in pre- 

 Cambrian time and that they were contemporaneous with the 

 metamorphism of the crystallines. This cannot be wholly the 

 case if the foregoing interpretation of the age of the sediments 

 is correct, since these igneous rocks are intruded into the rocks 

 here considered of Silurian age. 



The areal distribution of the formations enumerated above 

 is given in the accompanying map (Plate X) which has been 

 compiled from the published maps of the New York and Penn- 

 sylvania state surveys ; from the New York and Washington 

 folios of the U. S. Geological Survey; and the manuscript 

 maps of Dr. Bascom and the writer. The scale of the map 

 has required more or less distortion in the representation of 

 the quartzite and limestones and in the mapping of the rocks 

 of the less metamorphosed western portion of the plateau. 

 The lines must therefore be regarded as only provisional, espe- 

 cially in the areas north of Washington and that part of Penn- 

 sylvania which has not been mapped by Dr. Bascom. 



General Structural Lines. 



The broad structure lines of the Maryland Piedmont, so far 

 as they have been worked out in the eastern or more crys- 

 talline portion, are indicated in the accompanying sketch by 

 lines occupying the axes of the synclines and anticlines. It is 

 impossible on any small scale map to represent the minor 

 structural lines, which of course more or less modify the posi- 

 tion of the axes of the major folds. It is believed, however, 



