CHAP. III. UPHEAVAL OF CENTRAL DISTRICT OF SCOTLAND. 47 



sion to the ' era of existing continents,' a period supposed to 

 have coincided in date with the first appearance of man upon 

 the earth, since which event it was imagined that the relative 

 level of the sea and land had remained stationary, no im 

 portant geographical changes having occurred, except some 

 slight additions to the deltas of rivers, or the loss of narrow 

 strips of land where the sea had encroached upon its shores. 

 But modern observations have tended continually to dispel 

 this delusion, and the geologist is now convinced that at no 

 given era of the past have the boundaries of land and sea, or 

 the height of the one and depth of the other, or the geogra 

 phical range of the species inhabiting them, whether of animals 

 or plants, become fixed and unchangeable. Of the extent to 

 which fluctuations have been going on since the globe had 

 already become the dwelling-place of man, some idea may be 

 formed from the examples which I shall give in this and the 

 next nine chapters. 



Upheaval since the Human Period of the Central 

 District of Scotland. 



It has long been a fact familiar to geologists, that, both on 

 the east and west coasts of the central part of Scotland, there 

 are lines of raised beaches, containing marine shells of the 

 same species as those now inhabiting the neighbouring sea.* 

 The two most marked of these littoral deposits occur at 

 heights of about forty and twenty-five feet above high-water 

 mark, that of forty feet being considered as the more ancient, 

 and owing its superior elevation to a longer continuance of 

 the upheaving movement. They are seen in some places to 

 rest on the boulder clay of the glacial period, which will be 

 described in future chapters. 



* R. Chambers, ' Sea Margins ; ' Jordan Hill, Mem. Wern. Soc. vol. 

 1848, and papers by Mr. Smith of viii., and by Mr. C. Maclaren. 



