IMPORTANCE OF THEORY. I2Q 



had a striking instance of how easy it is to 

 overlook phenomena, however conspicuous, be- 

 fore they have been observed by any one. We 

 spent many hours at Cwm Idwal, examining 

 the rocks with extreme care, as Sedgwick was 

 anxious to find fossils in them, but neither of 

 us saw a trace of the wonderful glacial phe- 

 nomena all around us; we did not notice the 

 plainly scored rocks, the perched boulders, the 

 lateral and terminal moraines, yet these phe- 

 nomena are so conspicuous that, as I declared 

 in a paper published many years afterwards in 

 the Philosophical Magazine (1842), a house 

 burnt down by fire did not tell its story more 

 plainly than did this valley. If it had been 

 filled with a glacier, the phenomena would have 

 been less distinct than they now are." 



It may seem strange that two men, one already 

 a famous geologist and the other soon to be- 

 come one, should have overlooked such evi- 

 dence, which has since become so interesting, 

 so widely known, and so well understood. The 

 secret of their failure is that they were not 

 looking for it. It is usually the things that 

 men look for that they see; and to look for 

 things as yet unseen requires a theory as a 

 headlight. Even if they had noticed the mate- 

 rial of the moraines, the perched boulders, and 

 9 



