124 THE WONDERFUL CENTURY. CHAP. xn. 



The most interesting and instructive erratic blocks are 

 those found upon the slopes of the Jura, because they 

 have been most carefully studied by Swiss and French 

 geologists, and have all been traced to their sources in 

 the Alpine chain. The Jura mountains consist wholly 

 of Secondary limestones, and are situated opposite to the 

 Bernese Alps, at a distance of about fifty miles. Along 

 their slopes for a distance of a hundred miles, and ex- 

 tending from their base to a height of 2000 feet above 

 the Lake of jSTeuchatel, are great numbers of rocks, some 

 of them as large as houses, and always quite different 

 from that of which the Jura range is formed. These 

 have all been traced to their parent rocks in various parts 

 of the course of the old glacier of the Rhone, and, what 

 is even more remarkable, their distribution is such as to 

 prove that they were conveyed by a glacier and not by 

 floating ice during a period of submergence. The rocks 

 and other debris that fall upon a glacier from the two 

 sides of its main valley form distinct moraines upon its 

 surface, and however far the glacier may flow, and how- 

 ever much it may spread out where the valley widens, 

 they preserve their relative position so that whenever 

 they are deposited by the melting of the glacier those 

 that came from the north side of the valley will remain 

 completely separated from those which came from the 

 south side. It was this fact which convinced Sir Charles 

 Lyell that the theory of floating ice, which he had first 

 adopted, would not explain the distribution of the er- 

 ratics, and he has given in his " Antiquity of Man " (4th 

 ed., p. 3-44) a map showing the course of the blocks as 

 they were conveyed on the surface of the glacier to their 

 several destinations. Other blocks are found on the 



