78 TRAXSACTIOXS OF THE [XOY. 30, 



and believed in regard to that real but intangible medium 

 throngli the undulations of which light traverses space. 



The second paper is an attempt to estimate tlie annual erosion 

 over the surface of the two American continents by atmospheric 

 agencies. Compiling all the observations made, Mr. Reade's con- 

 clusion is that rain and rivers remove, in mechanical suspension, 

 one foot of the surface in 6,000 years. But to this measure of 

 denudation, which is based upon the sediment transported by 

 different streams, he adds an estimate of the matter carried away 

 in chemical solution, which he supposes to be about one-third of 

 that transported in suspension. Combining both, he estimates that 

 for the entire continents about one foot Avould be removed from 

 the surface in 4,500 years. However, it should be said that, in 

 the better watered portions, where the agents of erosion are more 

 active, one foot is removed from the surface in about one-half 

 that time, or from 1,800 to 2,500 years. 



In Professor Hull's article, which is printed in the Transac- 

 tions of the Royal Geological SocieUj of Irehuid, 1885, an effort 

 is made to trace the origin of many of the sedimentary rocks of 

 Europe and North America to the erosion of a broad continent, 

 which, in his opinion, formerly occupied the greater part of the 

 area of the North Atlantic Ocean, and from which eroded ma- 

 terial was carried into the sea basins, which, he imagines, then 

 occupied the places of Europe and eastern North America. 



To sustain his theory, he quotes the facts reported by many 

 geologists, that the mechanical sediments — conglomerates, sand- 

 stones, and shales — of the Palaeozoic series, which have, along 

 the Alleghany belt, a thickness of 20,000 feet, diminish to 4,000 

 feet in the valley of the Mississippi. 



A better acquaintance with American geology would have 

 saved Professor Hull from an error into which he has here fallen. 

 It is true that the strata which have been derived from the ero- 

 sion of the land do increase from the center of the Mississippi 

 valley to the Alleghany belt; but there they are at their maxi- 

 mum, because this was their place of origin. Among these strata 

 may be mentioned the " Serai conglomerate " of Rogers (" Mill- 

 stone grit"), which is at some localities in Pennsylvania 1,000 

 feet in thickness, and some of its pebbles aie six inches or more 



