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



sion to the "era of existing continents," a peinod supposed 

 to have coincided in date with the first ajipearance of man 

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

 relative level of the sea and land had remained stationary, 

 no important 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 geographical range of the species inhabiting them, 

 whether of animals or plants, become fixed and unchange- 

 able. 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 neighboring 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 fort}' 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. viii., 

 18-18, and papers by Mr. Smith of and by Mr. C. Maclaren. 



