108 INDEX TO THE STRATIGEAPHY OF NORTH AMERICA. 



The volcanic origin of these ancient rocks is clearly shown by flow banding, amygdaloidal 

 structure, and spherulites, as described by Williams and Bascom. The greenstones are. sheared 

 dense rock, veined with asbestos and chlorite. The original structure is seldom preserved, but 

 the rock is apparently an altered basalt. The rhyolitic rocks are of purple and red tints, often 

 porphyritic and frequently banded by flow structure of spherulitic streaks. The rhyolitic rocks 

 predominate in this area and apparently overlie the greenstone, for the basal Cambrian sediments 

 are composed largely of rhyolitic fragments and not of basaltic detritus, as would be the case 

 if the greenstone were younger and had been eroded from most of the area. 



Basal sandstones. — Overlying these softer rocks are about 4,600 feet of sandstone, quartzite, 

 and shale of Georgia,n (Lower Cambrian) age. The basal beds, forming the higher and more 

 rugged portions of the mountains, are composed of coarse purple and yellowish banded sand- 

 stones, fine conglomerate, and arkose, with white feldspathic and vitreous sandstones above. 

 The purphsh conglomerate bed is composed of small pebbles and grains of quartz, feldspar, 

 and purplish slate or tuff, the flat slaty fragments often having a diameter of 2 inches. This 

 grades almost imperceptibly through soft purplish arkose into the reddish rhyolitic eruptives 

 below, demonstrating their derivation largely from similar volcanic rocks exposed along the 

 near-by shore of the Georgian sea. In Maryland, and also at Moimt Holly, at the north end 

 of South Mountain, basal conglomerates contain numerous large quartz pebbles, probably in 

 part derived from the granitic basement complex of the Piedmont. 



This basal sandstone, on account of its hardness, forms high, rugged ridges in the heart of 

 the range, such as Rocky Mountain and Snowy Mountain. It is continuous with the Weverton 

 sandstone of the Catoctin and South mountains, Maryland, as mapped by Keith, and the name 

 is therefore used here. The underlying Loudoun formation, which is described by Keith as 

 variable in composition and thickness in Maryland, was not recognized as a distinct formation 

 in this area but may be represented in the soft arkose at the very base of the sedimentary series. 



Upper sJmles and sandstones. — ^Above the Weverton sandstone there are about 3,200 feet of 

 shale and soft sandstone in which are two horizons of hard, ridge-making sandstone. The 

 softer beds are poorly exposed, being everywhere covered by the debris from the adjacent sand- 

 stones. Their presence is inferred from the fact that their outcrop is always occupied by valleys 

 and depressions. Their character is indicated in part by occasional fragments of thin shaly 

 sandstone and black banded slate or red ferruginous shale. The hard sandstone beds form the 

 ridges along the mountain front and cap the high, flat-tipped Sandy Ridge, as weU as Big Flat 

 Ridge north of Fayettevflle. The lower of the two sandstones is the more massive and is 

 composed of a hard quartzitic stratum, usuaUy of dark-gray color and veined with quartz and 

 a softer, granular, white layer containing long, slender scolithus tubes. The upper hard bed at 

 the top of the shale is a milk-white or slightly pinkish granular calcareous sandstone, frequently 

 disintegrating by the removal of the soluble cement to yellowish quartz sand, which is quarried 

 for building purposes. This bed also contains numerous Scolithus linearis borings, and in 

 places Camarella minor and fragments of Oleneflus have been found, by which its age has been 

 determined to be Georgian (Lower Cambrian) . 



In the Catoctin and South mountains of Maryland Keith has mapped above the Weverton 

 sandstone 800 to 1,200 feet of shale (Harpers) and 500 to 700 of sandstone (Antietam). The 

 Harpers shale is typically exposed at Harpers Ferry, on the Potomac River, and, as described 

 by Keith, consists of a bluish-gray shale with a few thin sandstone beds. Northward these 

 sandstone beds are said to thicken, some attaining 50 feet, but do not have an appreciable 

 effect on the topography. On the road from Monterey to Waynesboro, in the southeast corner 

 of the area shown on the map, this series is fairly weU exposed, but, according to Keith, the 

 structure is complicated by folding and faulting. Above the Weverton sandstone in this sec- 

 tion, as seen by the writer, are shales or slates, in part dark-banded, containing a conspicuous 

 white scolithus-bearing sandstone 20 to 30 feet thick, all of which is mapped by Keith as 

 Harpers shale. Above the shale is the scolithus sandstone in which Walcott' found OleneUus and 

 CaTuarella minor, as noted above, and which is mapped by Keith as Antietam sandstone. 



