242 SCIENCE. [x. 



scientific man, but by a colonel in the Guards, known 

 to fame only as one of the most perfect horsemen in the 

 world. 



Let me illustrate my meaning by an example. A 

 man I do not say a geologist, but simply a man, 

 squire or ploughman sees a small valley, say one of 

 the side-glens which open into the larger valleys in 

 the Windsor forest district. He wishes to ascertain 

 its age. 



He has, at first sight, a very simple measure that 

 of denudation. He sees that the glen is now being 

 eaten out by a little stream, the product of innumer- 

 able springs which arise along its sides, and which are 

 fed entirely by the rain on the moors above. He finds, 

 on observation, that this stream brings down some 

 ten cubic yards of sand and gravel, on an average, 

 every year. The actual quantity of earth which has 

 been removed to make the glen may be several million 

 cubic yards. Here is an easy sum in arithmetic. 

 At the rate of ten cubic yards a-year, the stream 

 has taken several hundred thousand years to make 

 the glen. 



You will observe that this result is obtained by 

 mere common sense. He has a right to assume that 

 the stream originally began the glen, because he finds 

 it in the act of enlarging it ; just as much right as he 

 has to assume, if he find a hole in his pocket, and his 

 last coin in the act of falling through it, that the rest 

 of his money has fallen through the same hole. It is 

 a sufficient cause, and the simplest. A number of 

 observations as to the present rate of denudation, and 

 a sum which any railroad contractor can do in his head, 

 to determine the solid contents of the valley, are all 



