REVIEWS — THE STORY OF A BOULDER. 495 



Other theories speedily sprang up, endeavouring to account for the phenomena 

 by supposing great bodies of water rushing with terrific force across whole con- 

 tinents, sweeping away the tops of hills, tearing up and dispersing entire geologi- 

 cal formations, and strewing the ocean-bottom with scattered debris. But this 

 explanation had the disadvantage of being wofully unphilosophical and not very 

 clearly orthodox. Such debacles did not appear to have ever taken place in any 

 previous geologic era, and experience was against them. Besides, they did not 

 account for some of the most evident characteristics of the phenomena, such as 

 the northern character of the formation, the long parallel striations of the rock, 

 surfaces, and the perching of huge boulders on lofty hills, often hundreds of miles 

 distant from the parent rock. Geologists were completely at fault, and the 

 boulder-clay remained a mystery for years. 



"When we consider the physical aspect of the countries where the question was 

 studied, we cannot much wonder that the truth was so hard to find. In the midst 

 of corn-fields and meadows, one cannot readily realize the fact that the spot 

 where they stand has been the site of a wide-spread sea ; and that where now 

 villages and green lanes meet the eye, there once swam the porpoise and the 

 whale, or monsters of a still earlier creation, unwieldy in bulk and uncouth in 

 form. Such changes, however, must have been, for their traces meet us on every 

 hand. We have the sea dashing against our shores, and there seems nothing at 

 all improbable in the assertion that once it dashed against our hlll-tops. No one, 

 therefore, has any difficulty in giving such statements his implicit belief. But 

 who could have dreamed that these fields, so warm and sunny, were once sealed 

 in ice, and sunk beneath a sea that was cumbered with many a wandering ice- 

 berg? Who could have imagined, that down these glens, now carpeted with 

 heath and harebell, the glacier worked its slow way amid the stillness of perpetual 

 snjw ? And yet, strange as it may seem, such is the true solution of the prob- 

 lem. The boulder-clay was formed during the slow submergence of our countr- 

 beneath an icy sea, and the rock surfaces owe their polished and striated appear- 

 ance to the grating across them of sand and stones frozen into the bottom of vast 

 icebergs, that drifted drearily from the north. That we may better see how these 

 results have been effected, let us glance for a little at the phenomena observable 

 in northern latitudes at the present day. 



Icebergs are formed in three principal ways : — 1st. By glaciers descending to 

 the shore, and being borne seawards by land-winds ; 2nd. By river-ice packed 

 during spring, when the upper reaches of the rivers begin to thaw; 3rd. By 

 coast-ice. 



I. There is an upper stratum of the atmosphere characterised by intensecold 

 and called the region of peipetual snow. It covers the eai th like a great arch, 

 the two ends resting, one on the arctic, the other on the antarctic zone, while the 

 centre, being about 16,000 feet above the sea,* rises directly over the tropics, 



*The average height of the snow-line within the tropics is 15,207 feet, but it varies accord ■ 

 ing to the amount of land and sea adjacent, and other causes. ThiTS. among the Bolivian 

 Andes, owing to the extensive radiation, and the ascending cuirents of air from tlie neigh- 

 bouring plains and valleys, the line stands at a level of 18,000 feet, while, on mountains near 

 Quito, that is, immediately on the equatorial line, the lowest level is 15,795.— See Mrs. 

 Somerville's Physical Geography, 4th edit. p. 314. 



