PURPOSE OF THE PAPER 303 



and later formations, including the Mississippian sandstones of Little 

 North Mountain. The dikes closely parallel each other and can be traced 

 for long distances across the Valley group of limestones and the later 

 rocks of the Alleghany Eidges to the west, but they have not been traced 

 farther east than the northwest base of the Blue Eidge. The rocks are 

 of especial interest, since several types mark the first occurrence yet noted 

 in the southeast Atlantic States. 



The purpose of this paper is to record the distribution, occurrence, and 

 relations of the dikes; describe their microscopic petrography, based on 

 the study of many thin-sections; discuss their chemical composition, 

 based on complete chemical analyses of the different types, and from their 

 calculated norms indicate the position of the rocks in the quantitative 

 system. 



The writers do not claim that this study includes the discovery and 

 location of every dike or exposure of igneous rock within the limits of 

 the region ; on the contrary, we are convinced that other dikes will prob- 

 ably be found. It is believed, however, that the investigation has been 

 sufficiently detailed to warrant the statement that the discovery of addi- 

 tional types of igneous rocks within the limits of the region is remote, 

 and that in the event of additional dikes being discovered, which is more 

 than probable, they will be found on careful petrographic study to belong 

 to one of the several types recognized and described in this paper. 



For the complete and accurate chemical analyses of the rocks described 

 in this paper the writers wish to make grateful acknowledgment to Mr. 

 J. Wilbur Watson and Mr. J. G. Dinwiddie, instructors in chemistry in 

 the University of Virginia. 



Resume of the Geology of middle western Virginia 



No attempt is made here at a complete discussion of the geology of 

 middle western Virginia, 6 nor does the present problem warrant it. Only 

 those features of the general geology, therefore, which are essential to the 

 understanding of the relations of the igneous materials to the geology of 

 the region will be considered. Structurally, the geology of the region is 

 quite varied and in places complex because of faulting associated with 

 folding. 



The rocks of the region into which the dikes of igneous material have 

 been intruded are entirely of sedimentary origin and include strata rang- 

 ing in. age from Lower Cambrian to Mississippian (Lower Carbonifer- 



8 For a more complete discussion of this regiOD the reader is referred to Bulletin U a 

 Of the Virginia Geological Survey, 1900, 



