328 J. F. KEMP ISOSTASY AND APPLIED GEOLOGY 



beyond peradventure that coal is of vegetable origin, and that in the vast 

 majority of cases its parent vegetation has grown where, on falling down 

 or otherwise accumulating, the carbonaceous matter was buried beneath 

 a protecting cover of fresh water. 



Yet the swamps of necessity flourished along an irregularly subsiding 

 shoreline, such that, after slow accumulation, marine waters or great 

 bodies of fresh water could, when suddenly accelerated subsidence took 

 place, rush quickly over the swamp and bury its peat under rapidly grow- 

 ing fragmental sediments. Sometimes the rush was very rapid. The 

 upstanding tree trunks in the Nova Scotia Coal Measures, which Sir J. 

 William Dawson described many years ago, were entombed before they 

 even fell over. In the hollow stumps of some the very insects which in- 

 habited them were fossilized. After the rush a pause often ensued and a 

 new swamp started. The peat accumulated again until renewed subsi- 

 dence, with the inrush of fragmental sediments, buried it as before. We 

 can only interpret our coal strata as those characteristic of flat coastal 

 plains, with swamps of fresh water and a not very distant ocean or huge 

 body of fresh water. 



It is reasonable to conclude that along the coastline behind which the 

 coal swamps grew deltas and offshore deposits were rather rapidly accu- 

 mulating. During the time required for the accumulation of the peat 

 for a coal seam the rigidity of the local crust of the earth held the rela- 

 tions steady. When the accumulating load overcame the rigidity and the 

 shore sank, the raw material of the coal seam was submerged and buried. 

 In the broad range of geological sections there are few which furnish such 

 good illustrations of what one might almost call the normal progress of 

 isostatic adjustment. In the end, however, coal seams almost always 

 come to rest in a series of gentle or more violent folds, such as might 

 result when the full oscillatory course had been run and isostatic balance 

 had been so .seriously disturbed by offshore sedimentation as to lead to 

 final crumpling. 



Illustrations of isostatic Adjustment in engineering Work 



Extremely suggestive illustrations on a small scale are often afforded 

 when engineers make a fill for a railway embankment on a soft support. 

 The well known Lucine cut-off filled in by the Southern Pacific Eailroad 

 across an arm of the Great Salt Lake is an instance, but many others are 

 known. In former years the spoil from various excavations in New York 

 City was used as fill on the salt meadows of Pelham Bay Park. It pro- 

 duced from the soft turf and mud beautiful anticlines and synclines for 



