The Lifting of Blue Mountain. 87 



layer, sometimes of one kind of sediment and sometimes of 

 another. These are now found as la3^ers of sandstone, limestone, 

 coal, shale, slate and various combinations of sandstone, shale, 

 etc. ^^'"ith the close of the first great age (Paleozoic) in sedi- 

 nientation in the Appalachian trough, the earth's forces again 

 became active, and sufficient pressure was exerted from the 

 Atlantic coast side of the continent to raise this great mass of 

 sediments above the sea and to fold it in ridges and hollows, 

 very much as layers of paper or cloth would fold from pressure 

 applied to the edges of the layers if they were partially confined 

 above and below. This Avas varied, however, in the great rock- 

 masses by the frequent shearing on the line of the folds and the 

 thrusting of masses of rock one over the other, as cards shift 

 over each other under pressure. One of these folds, with minor 

 folds within it, has by subsequent agencies been carved into 

 the Blue ridge. 



The epoch of folding Avas several millions of years ago ; so 

 long since that sufficient time has elapsed for thousands of feet 

 of sediments to be deposited in the interior lakes and seas of the 

 North American continent and for animal life to develop from 

 the then highest types of fish and reptile to the higher mammals, 

 at the head of which man stands today. 



During the thousands of centuries since the first great Appa- 

 lachian uplift, the rain, frost, and snow have been at work 

 sculpturing the old land surface and slowly working out the 

 mountains, valleys, and plains. It is not improbable that the 

 process of mountain uplift and that of wearing away the mount- 

 ains to a relatively level area (baselevel of erosion) may have 

 taken place several times, the intervals of rest between the wear- 

 ing away of the highland and mountains and the succeeding 

 epoch of uplift being of long duration — so long, in fact, that 

 centuries might pass without effecting a marked change in the re- 

 lations of the land and sea. 



It was not far back, geologically speaking, that the Blue ridge 

 was a part of, and not distinct from, a great plain that was 

 broken by low hills and valleys and drained by streams flowing 

 into a river that occupied relatively the same position that the 

 Potomac does now. The continent was then at a lower level in 

 relation to the sea, and it Avas not until it became elevated that 

 the Potomac began to cut doAvn into its bed in the old plain and 

 carry out to the ocean the material Avhich filled the areas now 



