230 Pogue, Jr. — Geology and Structure of Volcanic Rocks. 



plagioclase ; 35*3 per cent angite ; 17'4 per cent olivine ; and 

 1*7 per cent magnetite. 



The analysis, made by Dr. A. S. Wheeler, is : 



Si0 2 Al a 3 Fe 2 3 FeO MgO CaO Na 2 K 2 H 2 C0 2 Total 

 47-G6 19-24 1-83 8*67 10-79 9-91 1-14 0'26 0-06 0-00=99-56 



From this its position in the quantitative system is calculated 

 to be: Class III, salfemane; order 5, perfelic; rang 5, per- 

 calcic ; subrang, not needed. The rock corresponds to 

 Tcedabekase. 



The diabase is the youngest rock in the district and is prob- 

 ably of Triassic age. As is well known, dikes of fresh 

 olivine diabase have a widespread occurrence throughout the 

 Piedmont Plateau, and in many places may be traced into 

 areas of Triassic sandstone. 



Structured features. 



Folding. — The region has been squeezed into great folds 

 during a period of severe compression, the most evident effect 

 of which has been the mashing of many of the rocks into 

 schists. The folds may not be directly observed, but their 

 presence is inferred from three concurrent lines of evidence. 



1. Bedding planes, which indicate a former horizontal 

 extent, often depart from this direction, and have a variable 

 dip either to the northwest or to the southeast, and at times 

 are even vertical. Were these sufficiently well preserved, they 

 alone would indicate the exact nature of the folding; but they 

 are much obscured by schistosity and weathering, so that only 

 here and there can a measurement be obtained. Certain 

 generalizations, however, may be made. Bedding planes are 

 predominantly horizontal along certain northeast-southwest 

 lines in massive formations ; and tend to be vertical or nearly 

 so in the badly mashed belts. 



2. The surface outlines of the formations, best seen on the 

 geologic map, are indicative of folding. In general, the forma- 

 tions may be divided into two classes : first, those which 

 appear upon the surface as long, narrow stripes, which gradually 

 pinch out at the ends and never end abruptly against other 

 formations ; and second, those which occur in broad lenses and 

 oval areas, of little or no elongation, and often ending abruptly 

 against other formations. Man}' of the narrow bands are flow 

 rocks or tuffs and breccias, which must have been deposited in 

 layers or beds of horizontal extent. Their surface outline 

 seems to preclude any other possibility than that they are the 

 upturned edges of such beds, which now intersect the surface 

 vertically, or nearly so. Broader lenses and oval areas, 

 although often composed of the same rock as the narrow stripe, 



