Professor Forbes, on the Leading Phenomena of Glaciers. 251 



case of glaciers. We cannot, therefore, admit it to be any 

 sufficient argument* against the extension of ancient glaciers to 

 the Jura, for example, that they must have moved with a super- 

 ficial slope of one degree, or, in some parts, even of a half or a 

 quarter of that amount, whilst in existing glaciers the slope is 

 seldom or never under 3°. The declivity requisite to insure a 

 given velocity, bears a simple proportion to the dimensions of 

 a stream. A stream of twice the length, breadth^ aiid depth 

 of another, will flow on a declivity half as great, and one of 

 ten times the dimensions upon 1-lOth of the slope, t 



Poets and philosophers have delighted to compare the course 

 of human life to that of a river ; perhaps a still apter simile 

 might be found in the history of a glacier. Heaven-descended 

 in its origin, it yet takes its mould and conformation from the 

 hidden womb of the mountains which brought it forth. At 

 first soft and ductile, it acquires a character and firmness of 

 its own, as an inevitable destiny urges it on its onward career. 

 Jostled and constrained by the crosses and inequalities of its 

 prescribed path, hedged in by impassable barriers which fix 

 limits to its movements, it yields groaning to its fate, and still 

 travels forward seamed with the scars of many a conflict with 

 opposing obstacles. All this while, although wasting, it is re- 

 newed by an unseen power, — it evaporates, but is not consumed. 

 On its surface it bears the spoils which, during the progress 

 of existence, it has made its own ; — often weighty burdens de- 

 void of beauty or value, — at times precious masses, sparkling 

 with gems or with ore. Having at length attained its greatest 

 width and extension, commanding admiration by its beauty 

 and power, waste predominates over supply ; the vital springs 

 begin to fail ; it stoops into an attitude of decrepitude ; — it 

 drops the burdens, one by one. which it had borne so proudly 

 aloft, — its dissolution is inevitable. But as it is resolved into 

 its elements, it takes all at once, a new and livelier, and dis- 



* Elie de Beaumont, Annates des Sciences Geologiques par JRiviere, 1842. 



t This results approximately from the formulae of Dubuat and £ytelwein,~th« 

 velocity varies as the square root of the slope, and as the quare root of the mean 

 hydranlic depth. 



