68 James Geikie — On Changes of Climate. 



other on the Hoosac Mountain (Massaeliusetts) reaches an elevation 

 of 2022 feet. In the valley of the Connecticut river a raised beach 

 occurs at 1082 feet above the sea. Many of the raised beaches are 

 strewed with huge boulders, as if these had been stranded by rafts 

 of ice.' 



During the re-elevation of the land beds of clay accumulated 

 off the coast, and became gradually stocked with shells of an arctic 

 type. These are the " Leda clays " of Labrador and Maine, so ably 

 described by Dr. Dawson, Dr. Packard, and others. It can hardly 

 be doubted that they are the equivalents of the Scottish and Scan- 

 dinavian shelly clays. The fossils which they contain are very 

 decidedly Arctic in the lower beds, but in the upper beds they 

 give evidence of a gradually ameliorating climate. 



Dr. Packard seems, if I follow him rightly, to be of opinion that 

 the Leda clay is older than the osar, and Principal Dawson inclines, 

 but with some hesitation, to 'the same belief. It may appear pre- 

 sumptuous in me to differ from these authorities, yet after carefully 

 considering what they have written, I venture to think that the 

 evidence in support of their conclusions is hardly satisfactory. The 

 shelly clays (like those of Scotland) are sometimes covered with 

 deposits of sand and gravel (Saxicava Sands), but there is no proof 

 that these beds are -necessarily of the same age as the osar of the 

 interior of America. In America, as with us, the shelly clays are 

 _ confined to the maritime regions, and I have found no mention 

 made of osar or mounds and ridges of sand and gravel overlying 

 them. When it is remembered also that erratics everywhere cap 

 the sand and gravel ridges of the interior, and occur abundantly in 

 the fossiliferous clays of the maritime regions,^ while they may be 

 said to be absent from the interior of the osar, we can hardly, I think, 

 escape from these conclusions, — first, that the accumulation of the 

 osar took place under a milder condition of climate than charac- 

 terized the deposition of the shelly clays ; and, second, that of the 

 two deposits the osar must be the older. But of course it is quite 

 possible that some of the osar adjoining the maritime regions may 

 have been formed contemporaneously with the Leda clay, with 

 which some of the old «ea-beaches, at all events, must be syn- 

 chronous. If, therefore, we refer the accumulation of the American 

 osar to the period of subsidence, and the deposition of the " Leda 

 clay" to the following period of re-elevation, we shall have for North 



1 There is some uncertainty as to the height reached by the sea during the period 

 of subsidence that followed upon the retirement .of the ice-sheet. Perched blocks are 

 not always safe guides, as these may sometimes have been stranded along the sides of 

 mountains by glaciers. In many, or rather in most cases, howeyer, they would appear 

 to have been .carried by rafts of ice and dropped into their present positions. They 

 seem to give evidence, therefore, that the land subsided to at least 2600 feet below the 

 present level of the sea. But Dr. Packard thinks that some of the high-level terraces 

 described by Hitchcoci; are not of marine but freshwater origin, and that they are 

 relics of glacial lakes. In this case these terraces would resemble the parallel roads 

 of Glenroy. 



2 So much so indeed as to entitle them to be called " Boulder-clays." They are 

 .more or less distinctly stratiiied, however. (Packard.) 



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