55 2 THE JOURNAL OF GEOLOGY. 



ward. Mountains were therefore born of sea-margin deposits. 

 This view is entirely confirmed by the character of mountain 

 sediments. We have seen that these are coarsest near the crest, 

 becoming finer and then changing into limestones as we pass 

 farther and farther away from the crest. Now this is exactly 

 what we find in off-shore deposits. They are coarse sands and 

 shingle near shore, and then become progressively finer seaward, 

 until in open sea beyond the reach of even the finest mechanical 

 sediments, they are replaced by organic sediments which form 

 limestones. It seems evident, therefore, that the place of a 

 mountain-range before mountain-birth was a marginal sea- 

 bottom receiving abundant sediment from a contiguous conti- 

 nental land-mass. This explains at once the usual position of 

 mountains on the borders of continents. Here, then, is one 

 important point gained. 



But such enormous thickness as we often find would be 

 impossible unless the conditions of sedimentation on the same 

 spot were continually renewed by pari passu subsidence of the 

 sea-bottom. And we do indeed find abundant evidence of such 

 pari passu subsidence, not only at the present time in places 

 where abundant sediments are depositing, but also in the strata 

 of all mountain ranges. In the 40,000 feet thickness of Appa- 

 lachian strata nearly every stratum gives evidence by its fossils, 

 of shallow water, and often by shore marks of all kinds, of very 

 shallow water. Therefore the place of mountains while in prepa- 

 ration, in embryo, before birth, was gradually subsiding, as if borne 

 down by the weight of the accumulating sediments, and continued 

 thus to subside until the moment of birth, when of course a con- 

 trary movement commenced. The earth's crust on which the 

 sediments accumulated was bent into a great trough, or what 

 Dana calls a Geo-Syncline. This is another important poined 

 gained. 



But let us follow out our logic. If the earth's crust yields 

 under increasing weight of accumulating sediments, then ought 

 it also to rise under the- decreasing weight of eroded land sur- 

 faces. If it sinks by loading it ought also to rise by unloading. 



