144 Mathews — Structure of the Piedmont Plateau. 



igneous, some as sedimentary masses intruded by igneous rock 

 to form so-called injection gneisses. All have agreed that these 

 banded gneisses are probably pre-Cambrian in age whatever 

 their origin. Usually in eastern Maryland they are separated 

 from other metamorphosed sedimentary rocks by igneous 

 masses, but in the vicinity of Baltimore and in the Philadelphia 

 area, as shown by Dr. Bascom, these banded gneisses imme- 

 diately underlie the quartzite. 



The Quartzite. — The quartzite is a fine-grained, somewhat 

 saccharoidal, thin-bedded quartzite of white-brown color. The 

 beds are usually separated by thin films of muscovite in small 

 sparkling flakes. On the surface of these mica-covered cleav- 

 age or bedding planes frequently occur black tourmaline crys- 

 tals which occasionally show evidence of movement along these 

 planes. Individual specimens of the quartzite when massive 

 may appear like the quartzose layers of the banded gneiss, but 

 usually the rock is easily distinguished in the field. It gen- 

 erally is found dipping at a rather steep angle and because of its 

 resistance to weathering is often a topographic feature of the 

 region. This is a relatively thin formation which varies much 

 in thickness from one to several hundred feet. Where this 

 quartzite has been studied in anticlines and synclines, both 

 normal and overturned, it appears to be younger than the 

 banded gneiss and older than the neighboring marble. 



The Marble. — The marble is a coarse-grained to medium- 

 grained impure dolomite in which the impurities have been 

 entirely recrystallized into silicates such as diopside, tremolite, 

 phlogopite, etc. It is almost always a dolomite in chemical 

 composition but may vary occasionally to a pure calcite rock. 

 The bedding in these rocks is not easily recognized, but it is 

 probable that the lines of impurity now represented by silicates 

 indicate original differences in the sedimentary deposits. The 

 amount of such impurities causes the rock to vary widely from 

 a pure carbonate to one so rich in silicates that it may easily 

 be mistaken for a gneiss until tested for hardness or with acid. 

 These beds in the marble show considerable variation in the 

 steepness of their dip, ranging from 10° up to 70° and occasion- 

 ally they are found in overturned anticlines and synclines. 

 The thickness of the formation is very variable and practically 

 indeterminate on account of the paucity of exposures. Its 

 presence is usually indicated by valleys along the sides of the 

 quartzite ridges and its thickness apparently varies from noth- 

 ing up to more than 2000 feet. JSTo fossils have been found in the 

 highly crystalline marbles and it is improbable that if originally 

 present they could have withstood the changes which this 

 rock has undergone.* The formation, however, has been traced 



* A few deformed chert nodules have "been found and they may yield micro- 

 scopic forms on closer examination. 



