316 



TRANSACTIONS OP THE 



and with this we perceive considerable discrepancy. 

 Taking the dimensions from that map of the respective 

 classes of rocks which cross our line of section, and com- 

 paring them with our own, they stand as follows : 



M^Clure. 



Primitive, 30 miles 



Old red sandstone, . . . . . 10 " 



Transition, . . ... . . 10 " 



Primitive again to the crest of the Blue Ridge, 6 " 



Transition again, . . . . . 15 " 



71 " 



T. and C. 



Class I. Primitive 10 miles, Gold Belt 11, 21 miles 

 II. Modified, amphibolic and mixed, 36 " 



III. Transition, .... 14 " 



IV. Secondary overlying Class I. in 



part, 



71 



The writer has elsewhere alluded to this supposed 

 belt of old red sandstone, and pointed out the true site 

 of that rock, 100 miles westward from the position assign- 

 ed to it by the authority before cited. 



In examining the data which we have brought together, 

 towards elucidating the geological structure of this coun- 

 try, we cannot but be struck with the enormous aggre- 

 gate thickness of those stratified rocks, which by being 

 placed as it were on their edges, side by side in almost 

 a vertical position, afford -the means of correctly mea- 

 suring their entire dimensions, and display within a 

 limited area a vertical depth far exceeding in amount any 

 estimate that we have hitherto had the means of forming. 



We subjoin the result. 



Below Fredericksburg, the primitive rocks are ob- 

 scured by more recent deposits ; but having twice trav- 



