Pogue, Jr. — Geology and Structure of Volcanic Bocks. 233 



the case of the present area, the higher segment or land mass 

 was to the northwest, whereas in the case of the Appalachians, 

 as is generally accepted, the higher segment or "hinterland" 

 of Sness w T as to the southeast. The district under considera- 

 tion, however, is too limited in size to afford widespread 

 generalizations on this point, unless corroborated in the future 

 by other observations in the slate belt. 



Jointing. — Joint planes are distributed throughout the dis- 

 trict in all formations, but are most abundant in areas of 

 massive rocks. In general, the degree of jointing decreases 

 with increased schistosity : the two features seem to be com- 

 plementary. Where bedding is horizontal, jointing is invariably 

 well developed. 



These features indicate that the jointing is largely the result 

 of the compressive force of folding. Rock masses situated on 

 the crests or in the troughs of folds escaped to a large degree 

 the effects of mashing, but in transmitting the force w T ere 

 themselves broken into blocks bounded by joint planes. 



The following conclusions are drawn from a graphical plot- 

 ting of the joints. The jointing is grouped into four impor- 

 tant sets, in their order of importauce, as follows : 



N. 5° W. to 1ST. 30° w. 

 N. 65° W. toN. 85° W. 

 N. 15° E. to N. 35° E. 

 N. 70° E. to N. 85° E. 



There is no important difference in the jointing in the slate 

 and in the other formations. In massive formations there are 

 no important joint planes parallel to cleavage planes in schistose 

 formations. There is no evidence for believing any important 

 jointing on crests of folds to be due to tension : joints from 

 such a cause would be parallel to the schistosity developed on 

 the limbs of the fold.* 



All jointing in the region is not considered the result of a 

 single period of compression. There is doubtless jointing, 

 also, from other subsequent earth movements. 



Faulting. — While there is at no place conclusive evidence 

 of faulting on an important scale, a number of probable faults 

 have been indicated on the geologic map. These cannot be 

 verified by field observation ; but their presence is suggested 

 by the abrupt ending of certain formations, as if cut off by 

 dislocations, and in cross-section by the failure of bands to be 

 repeated on the corresponding parts of folds. The smaller 

 faults have a general parallelism to one another, and to 

 Fourmile Branch, which pursues a remarkably straight course 



* Van Hise, C. R., Principles of North American Pre-Cambrian Geology. 

 16th Ann. Report, U. S. Geol. Survey, 1895, p. 669. 



