308 WATSON AND CLINE IGNEOUS DIKE? IN VIRGINIA 



zoic era. The tendency of dike control by joints and faults is a matter of 

 record in many cases and has been observed in near-by regions, namely, 

 in the Eichmond coal basin by Shaler and Woodworth 9 and in the granite 

 and sandstone areas of Xorth Carolina by Watson and Laney. 10 



The investigations by the writers of the Virginia region suggest to 

 them that the dikes which form the subject of this study have followed 

 fissures along lines of normal faulting and probably jointing which ex- 

 hibit the same direction of strike, developed contemporaneously with the 

 period of deformation resulting in the production of faults in the Tri- 

 assic areas of the eastern United States. Xormal faults in the sedimen- 

 tary rocks west of the Blue Eidge correlated by the writers with the Tri- 

 assic period of deformation have been observed, and further careful study 

 will doubtless reveal them in considerable numbers. In the Cambrian 

 areas immediately east of the Blue Eidge in Virginia, where conditions 

 are favorable for observation, normal' faulting in at least two directions 

 has been observed by the writers which equals in development that of the 

 Triassic areas near by. Evidence of the period of deformation which re- 

 sulted in the production of normal faults in these Cambrian areas indi- 

 cates to the writers that which was productive of similar structure in the 

 Triassic areas. It would seem probable, therefore, that normal faulting 

 of the Triassic beds was developed by disturbances more widespread in 

 the Appalachian region than has been previously supposed. 



Contact Mztamoephism 



Although thp dominant type of sedimentary rocks intruded by the 

 dikes is limestone, arid although the igneous bodies are of fairly good 

 size, measuring in some instances as much as 80 feet in width, no appar- 

 ent contact metamorphism has been noted. Angular fraorments of the 

 limestones, mostly of very small dimensions, were found incorporated in 

 the substance of some of the basic dikes, which are entirely unaltered and 

 without evidence of the slightest corrosion effect. Sharp contacts be- 

 tween the nepheline syenite and inclosing limestone were observed, where 

 minute dikelets (apophyses) of the syenite occupied fractures in the lime- 

 stone several inches in length, and small fragments of the limestone were 

 also incorporated in the syenite, but without the slightest manifestation 

 of alteration. 



* 19th Ann. Kept. I*. S. Geological Surrey, part ii. 1897-1898, p. - 

 10 North Carolina Geological Surrev. Bulletin No. ii. 1906. pp. 178-179. 182, 1S4-1S7. 

 241-243. 



