PETROLOGY OF A SERIES OF SHENANDOAH DIKES 683 



In addition to diabase, a series of complementary dikes of nepheline syenite, 

 camptonite, and monchiquite, intimately and interestingly associated with each 

 other, occur in the northern part of Augusta County, cutting the Shenandoah 

 group of limestones and the Mississippian rocks in Little North Mountain. The 

 dikes closely parallel each other and can be traced for long distances across 

 the valley limestone and related formations and the later rocks of the Alle- 

 gheny ridges to the west, but in no case have they been traced farther east 

 than the foothills of the Blue Ridge. Kinship of the three rock types is estab- 

 lished by particular characters in mineralogical and chemical composition. 

 Hydrochloric acid solution of each rock yields copious gelatinous silica on 

 evaporation. 



The rocks are of especial interest, since they mark the first occurrence yet 

 noted in the southeast Atlantic States. The geographic distribution and geo- 

 logic occurrence and relations of the dikes are described ; microscopical petrog- 

 raphy based on study of many thin sections, and chemical composition based 

 on complete analysis of the different rock types are discussed, and from their 

 calculated norms the position of the rocks in the quantitative system of classifi- 

 cation is determined. 



NORTHUMBERLAND VOLCANIC PLUG 

 BY H. P. GUSHING 



Published as pages 335-350 of this volume. 



Discussion 



Dr. L. V. Pirsson called attention to the fact that Dr. John S. Flett at the 

 meeting of the British Association for the Advancement of Science, at Dundee, 

 in the summer of 1912, read a paper on the petrology of Scotland, in which the 

 view was expressed that the pillow lavas represent a series of effusive rocks of 

 peculiar nature poured out under conditions of great stability and geologic 

 quiet and associated with sediments undisturbed at the time. If this view be 

 correct, it would indicate that the hypothesis of the lava having arrived at its 

 present place by thrusting may be correct. 



Prof. J. Volney Lewis : In some half a dozen localities in the Watchnng 

 Mountains of New Jersey a pillow structure very similar to that described by 

 Professor Cushing occurs in the upper parts of the Newark basalt flows. The 

 spheroidal masses are covered with an inch or two of glass, are imbedded in 

 glass, and are characterized by abundant radial cracking. These basalt flows 

 are associated with sediments in which no evidence of marine sedimentation 

 has yet been found and which are apparently of continental origin. It seems 

 impossible, therefore, to attribute the pillow structure to submarine flow. 



Dr. I. C. White : The failure to note the occurrence of this outcrop of Igne- 

 ous rock in New York by the older geologists and former State surveys has a 

 parallel in my own State of West Virginia. A narrow dike of peridotite occurs 

 in Pendleton County which remained unnoticed by William B. Rogers and his 

 assistants of the Virginia Survey until it was finally discovered by one of the 

 geologists of the United States Geological Survey half a century later. 



Another and more conspicuous occurrence of a peridotite dike cutting through 

 the great Pittsburgh coal bed and extending nearly to the top of the hills in 



