122 Dr. Charles Richetts — On Subsidence and Accumulation. 



the Boulder-clay lead to the inference that the more ancient valley 

 has to a great extent resulted from the eroding power of ice ; as 

 this arctic temperature continued the land subsided to a considerable 

 extent below the present sea-level, and whilst thus submerged the 

 sand and clay escaping from beneath the glaciers were spread over 

 what had become the bed of an extensive ice-bound bay. The 

 climate changing and, the glaciers becoming dissolved, the land, re- 

 lieved of its icy bm'den, became raised considerably above its present 

 level, and what is now our harbour was covered with a luxuriant 

 vegetation, the river running through it probably as a narrow 

 stream ; but as the sand and silt brought down by this and other 

 rivers were deposited in the bay, and as the accumulation became 

 greater, the influence of its pressure was extended more inland, so 

 that the valley on being depressed, instead of conveying a small 

 stream, became the lateral boundaries of the present estuary. 



There may be a difficulty in all cases of proving that the denuda- 

 tion of continents has been continuous, but of this there can be little 

 doubt respecting a large portion of North America. Those who 

 have perused the report of the United States expedition to the 

 Colorado river, will agree with Prof. Newberry in his conclusions 

 that the district could not have been submerged since the time when 

 the waters began to cut deeply into the Carboniferous limestones and 

 sandstones which constitute the Colorado plateau ; thus forming 

 those enormous gorges the Canons of the Colorado, having pei'pen- 

 dicular walls from 3000 to 6000 feet in height. Confirmation of 

 this opinion was afforded by Prof. Marsh in a tour made last year in 

 the Eocky Mountain region, and aloiig the tributaries of the 

 Colorado Eiver in Utah, where extensive freshwater lakes existed 

 during the Tertiary Period (Geol. Mag. 1871, p. 127). There- 

 fore the land situated at the same height on the opposite flanks 

 of the mountains must also, during the same time, have been far 

 above the sea-level, and must likewise during the whole of this ex- 

 tended period have been exposed to denudation from atmospheric 

 agencies alone. 



If the crust of the earth formed a rigid surface so that neither 

 elevation nor depression could take place, then, instead of deltas 

 which represent comparatively a minute portion of the enormous 

 amount which is known to have been removed from what constitute 

 the valley systems of large rivers, there would be enormous plains 

 extending for hundreds or rather thousands of miles, when the areas 

 exposed to denudation are so great as nearly half a million of square 

 miles in the case of the Ganges and Brahmapootra, or nearly a 

 million in that of the Mississippi and Missouri. 



Considering that the formation of deltas and bays is the result of 

 depression caused by the weight of sediment derived from the disin- 

 tegration of interior land surfaces brought down by rivers, it will 

 likewise be necessary to attribute the extension of the bays towards 

 the ocean to a previous existence of the same causes and that their 

 river-systems were formerly greatly extended, receiving as tribu- 

 taries rivers which now empty themselves into bays, and others 



