THE ICE AGE AND ITS WORK. 281 



The. most important of these facts relate to the erratic blocks from 

 tlie higher Alps, which are found on the flanks of the Jura Moun- 

 tains wholly formed of limestone, on which it is therefore easy to recog- 

 nize the granites, slates, and metamorphic rocks of the Alpine chain. 

 These erratic blocks extend along the Jura range for a distance of 100 

 miles, and up to a heiglit of li,015 feet above the Lake of Neufchatel. 

 The tirst important point to notice is that this highest elevation is 

 attained at a spot exactly opposite, and lu the same direction as, the 

 Rhone Valley, between Martigny and the head of the Lake of Geneva, 

 while north or south of this point they gradually decline in elevation 

 to about 500 feet above the lake. The blocks at the highest elevation 

 and central point can be traced to the eastern shoulder of Mont Blanc. 

 All those to the southwest come from the left hand side of the Lower 

 Rhone Valley, while those to the northeast are all from the left side of 

 the Upper Rhone Valley and its tributaries. Other rocks coining from 

 right-hand side of the Ui)per Rhone Valley are found on the right- 

 hand or Bernese side of the great valley between the Jura and the 

 Bernese Alps.* 



Now. this peculiar and definite distribution, which has been worked 

 out with the greatest care by numerous Swiss geologists, is a necessary 

 consequence of well known laws of glacier motion. The debris from 

 the two sides of the main valley form lateral moraines which, however 

 much the glacier may afterwards be contracted or spread out, keep 

 their relative position unchanged. Each important tributary glacier 

 brings in other lateral moraines, and thus when the combined glacier 

 ultimately spreads out in a great lowland valley the several moraines 

 will also spread out, while keeping their relative position, and never 

 crossing o\ er to mingle with each other. So soon as this definite posi- 

 tion of the erratics was worked out it became evident that the first 

 explanation — of a great submergence, during which the lower Swiss 

 valleys were arms of the sea and the Rhone glacier broke off in ice- 

 bergs, which carried the erratics across to the Jura — was altogether 

 untenable, and that the original explanation of Venetz and Charpen- 

 tier was the true one. - - - 



We must now consider brietiy the distribution of erratics in North 

 America, because they present some peculiar features and teach us 

 much concerning the possibilities of glacier motion. 



An immense area of the Northeastern States, extending south to 

 New York, and then westward in an irregular line to Cincinnati and 

 St. Louis, is almost wholly covered with a deposit of drift material, in 

 which rocks of various sizes are embedded, while other rocks, often of 

 enormous size, lie upon the surface. These blocks have been carefully 

 studied by the American geologists, and they present us with some 

 very interesting facts. Not only are the distances from which they have 

 been transported very great, but in very many cases they are found at 



* A map showing the lines of dispersiil of these erratics is ^iven in I^yell's Antiq- 

 uittj of Men, p. 344, and is reproiliicfil in my Island Life, p. 111. 



