Mathews — Structure of the Piedmont Plateau. 155 



much easier. It seems to be a southern extension of the 

 Chester Valley folding, 



Tocquan Anticline. — Tocquan anticline, recognized by Pro- 

 fessor Frazer in his report on Lancaster county (CCC, p. 128) 

 as a broad flat anticlinal arch somewhat tilted to the southeast, 

 crosses the Susquehanna river at McCall's Ferry. This struc- 

 tural line has been traced by the Pennsylvania geologists from 

 northern Chester county through Lancaster and York counties 

 to the Maryland line. Throughout this area the axis is com- 

 plicated by the presence of minor folds. It seems to rise to the 

 north and south. In Maryland the work along this axis has 

 not been carried very far, but the areal distribution of the rocks 

 seems to indicate that it corresponds roughly with the center 

 of the mica-schist zone passing east of Westminster, crossing 

 the Potomac to the southwest of Rockville. The occurrence 

 of interfolded phyllites along the western boundary of the 

 more crystalline part of the plateau, as worked out by Keith, 

 and of phyllites farther south to the west of Gaithersberg, sug- 

 gests that this axis is here even less strongly marked and that 

 it is broken into several shallow folds. 



The region between this anticlinal axis and the overlapping 

 Jura-trias bed on the west appears to be a shallow much-folded 

 synclinorium with an anticlinal axis passing near Union Bridge, 

 Maryland, but this region has not been worked with sufficient 

 detail to warrant more definite statements. 



On the eastern side of the Catoctin mountain the Shenandoah 

 limestone rises with an easterly dip from beneath the phyllites 

 and is in turn underlain by the Antietam quartzite and other 

 rocks of Cambrian age. 



Character and Distribution of the Igneous Rocks. 



The most prominent feature of the igneous rocks is the 

 occurrence of large masses of gabbro in more or less parallel 

 bands, separated for the most part by masses of granite and 

 smaller areas of ancient gneisses. The areal distribution of 

 these gabbro masses, which in the region about West Chester, 

 Pennsylvania, have been found by Dr. Bascom to extend across 

 from one belt to the other, is believed by the author to indicate 

 that the various occurrences represent what was once an 

 immense gabbro sheet extending from Laurel, Maryland, to the 

 Schuylkill river, a distance of fully 85 miles with an exposed 

 breadth of at least 15 miles. 



Through this great sheet of gabbro were apparently intruded 

 the granites and later meta-rhyolites and pegmatites, the whole 

 constituting one great series of igneous activity. When these 

 intrusions began or when they ceased is not clearly deter- 

 minable since contacts between the various rocks are almost 



