72 STUDIES, SCIENTIFIC AND SOCIAL chap. 



The most important of these facts relate to the erratic 

 blocks from the higher Alps, which are found on the 

 flanks of the Jura Mountains wholly formed of limestone, 

 on which it is therefore easy to recognise the granites, 

 slates, and old metamorphic rocks of the Alpine chain. 

 These erratic blocks extend along the Jura range for a 

 distance of 100 miles, and up to a height of 2,015 feet 

 above the Lake of Neufchatel. The first important point 

 to notice is, that this highest elevation is attained at a 

 spot exactly opposite, and in 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 south-west come from the 

 left-hand side of the lower Rhone valley, while those to 

 the north-east are all from the left side of the upper 

 Rhone valley and its tributaries. Other rocks coming 

 from the right-hand side of the upper 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 im- 

 portant 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 over to mingle with each other. So soon 

 as this definite position of the erratics was worked out it 

 became evident that the first explanation — by 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 

 1 See Map at p. 128. 



