COMMENCEMENT OT PRESENT "SPECIES. 71 



dnced by a quantity of loose blocks hurried along over 

 them by a flood. Another associated phenomenon is that 

 called crag and tail, which exists in many places, namely, 

 a rocky mountain, or lesser elevation, presenting on one 

 side the naked rock in a more or less abrupt form, and 

 on the other a gentle slope; the sites of Windsor, Edin- 

 burgh, and Stirling, with their respective castles, are spe- 

 cimens of crag and tail. Finally, we may advert to cer 

 tain long ridges of day and gravel which arrest the atten- 

 tion of travellers on the surface of Sweden and Finland, 

 and which are also found in the United States, where, 

 indeed, the whole of these phenomena have been observ- 

 ed over a large surface, as well as in Europe, It is very 

 remarkable that the direction from which the diluvial 

 blocks have generally come, the lines of the grooved rock 

 surfaces, the direction of the crag and tail eminences, and 

 that of the ciay and gravel ridges — phenomena, be it ob- 

 served, extending over the northern parts of both Europe 

 and America — are all from the north and north-west to- 

 wards the* south-east. We thus acquire the idea of a 

 powerful current moving in a direction from north-west 

 to south-east, carrying, besides mud, masses of rock which 

 furrowed the solid surfaces as they passed along, abrading 

 the north-west faces of many hills, but leaving the slopes 

 in the opposite direction uninjured, and in some instances 

 forming long ridges of detritus along the surface. These 

 are curious considerations, and it has become a question 

 of much interest, by what means, and under what circum- 

 stances, was such a current produced. One hypothetical 

 answer has some plausibility about it. From an investiga- 

 tion of the nature of glaciers, and some observations which 

 seem to indicate that these have at one time extended to 

 lower levels, and existed in regions (the Scottish High- 

 lands an example) where there is no perennial snow, it 

 has been surmised that there was a time, subsequent to 

 the tertiary era, when the circumpolar ice extended far 

 into the temperate zone, and formed a lofty, as well as 

 extensive accumulation. A change to a higher tempera- 

 ture, producing a sudden thaw of this mass, might set 

 free such a quantity of water as would form a large flood, 

 and the southward How of this deluge, joined to the di- 

 rection which it would obtain from the rotary motion of 

 the globe, would of course produce that compound or 

 south-easterly direction which the phenomena require 



