242 Geology of North Carolina. 



ished in a moment, and these walls stood forth confessedly 

 the work of God alone. They are nothing more than dykes 

 or veins of trap rock in granite, having that columnar struc- 

 ture so common in the rocks of this family : and being more- 

 over encrusted with a part of their own substance, partially 

 decomposed, which the imagination of an antiquary might 

 easily mistake for mortar. Although this rock agrees exactly 

 in external characters and composition with the classic Euro- 

 pean basalt, yet most of the distinguished French geologists 

 seem unwilling to pronounce them identical, from an appre- 

 hension that basalt always has a volcanic origin, and that no 

 evidence of volcanic action exists in the United States. We 

 are aware that there are several instances in geology, in 

 which we are obliged to regard rocks as specifically distinct, 

 although their external characters and composition are the 

 same ; because their origin was unquestionably different* 

 But in the case under consideration, we think European geol- 

 ogists ought not to require that we should show them extinct 

 craters of volcanos, before they will admit the igneous ori- 

 gin of our trap rocks. There may be other proofs of such an 

 origin quite as conclusive as this ; and if we are not deceiv- 

 ed, they are as numerous and decisive in regard to all the 

 members of the trap family in this country as in Europe. We 

 should, therefore, have no hesitation in pronouncing the rock 

 of the natural walls of Rowan to be genuine basalt : al- 

 though it may be of a more remote era than that in Europe. 

 We have seen no trap rock in our country, whose fracture 

 resembles the basalt of the Giant's Causeway so closely as 

 this. We subjoin Prof. O.'s description of some of these 

 dykes. 



Of the natural walls of Rowan, I made a particular exam- 

 ination of only two. The first is about four miles north of Sal- 

 isbury, and is known by the name of Jacobs' wall. The dykes 

 at this place, (of which their are several,) are narrow, running- 

 through a friable kind of granite. We uncovered one which 

 exhibited the following characters. 



Width — about eight inches, the sides being smooth planes,, 

 and separated from the granite by a thin crust of clay. 



Dip — eastward at an angle of 78 1-2 degress. 



Seamed — at right angles to the sides, dividing the whole into 

 very regular prisms, each crossing the wall, and consequently of 

 uniform length, but differing in diameter and in the number of 

 sides. The ends of these prisms being in the same plane, and 



