66 ON MR. M. READE's WORK ON MOUNTAIN BUILDING. 



measurements across tlie Alleghany chain ; the present 

 extent of the sedimentary strata, measured over hill and 

 dale, proves such a movement to have taken place ; i.e., 

 if the sedimentary strata were flattened out the}^ would 

 stretch several miles beyond the present breadth of that 

 chain. To which Mr. Reade's friends may reply — first, that 

 the heat which gave rise to the mountains expanded and 

 extended these strata ; second, the upthrust of archeean 

 rock has not onl}^ driven aside the sedimentaries, but also has 

 itself further expanded, and these expansions have so com- 

 plicated the measurements that they do not even proximately 

 represent the extent of the original beds. On the other 

 hand, Mr. Reade's theory does account for the undoubted 

 fact that a much greater surface is covered now by these 

 strata than is implied by a straight line across the chain, 

 which we suppose to be the approximate width of the 

 original beds. 



The effect of secular contraction on the crust is disposed 

 of by referring the whole to normal faults, which are found 

 everywhere, indicating that these evidences of tension strain 

 have affected the cnist pretty evenly throughout its entire 

 extent and thickness, sufficiently accounting for all the con- 

 traction which can reasonably be supposed to have taken 

 ]jlace since the formation of the solid crust. 



