1873.] 151 [Burbank. 



of a large portion of this region are strikingly similar to those of 

 northern Massachusetts, east of the Nashua Valley. We find there 

 the gneiss, passing by a gradual transition into mica and hornblende 

 schists on the one hand, and on the other into metamorphic granites, 

 sienites and greenstones, like those of northeastern Massachusetts. 

 Among these latter rocks especially, there are found vast numbers of 

 rounded, boulder-like masses lying partly buried in the soil. They 

 are, however, always of the same kind as the rocks in place, above 

 which they rest. I have observed enormous masses of this kind in 

 Wake County, between Wake Forest College and Rolesville. Some 

 of them far exceed in size any of the boulders of the New England 

 drift, and yet have the characteristic form of boulders, not at all re- 

 sembling the outcrop of ledges in the drift regions. 



These boulder-like forms are very numerous near the village of 

 Oxford, Granville County, where I studied them carefully. In this 

 locality they are very abundant in the small valleys or ravines formed 

 by the washing away of the loose material by the rains. In many 

 instances the boulder-like masses are scattered over the summits of 

 slight elevations, from which a portion of the material derived from 

 disintegration and exfoliation has been removed. 



That these rounded forms are not the result of attrition, but of chemi- 

 cal and atmospheric agencies acting upon rocks of a concretionary struc- 

 ture, is evident from their appearance and structure. 



Excellent illustrations of this concretionary structure are to be seen 

 in many places in the excavations along the line of the North Caro- 

 lina Railroad, in Guilford County. At one locality near Jamestown 

 the excavation was made through a little hill of sienite, about fifteen 

 feet in height above the grade of the road. The greater portion of 

 the rock is entirely disintegrated to a greater depth than the excava- 

 tion extended; but the central mass of the knoll is made up of con- 

 cretionary masses of sizes varying from five or six feet to less than 

 one foot in diameter. Some of the concretions have the form of 

 almost perfect spheres. When the excavation was made, one of 

 these masses was split through the centre, thus showing plainly the 

 concentric lines of incipient exfoliation. The loose, decomposed ma- 

 terial around and above these masses still retains its position as in 

 the original rock. 



The trap dikes of this region also afford fine examples of the pro- 

 duction of boulders of decomposition, as they have been aptly termed 

 by Professor Hartt. On the road between Oxford and Hillsboro', 

 near the bridge over Tar River, a trap dike which traverses the 



