140 



The Glacial Theory, 



ciers. We see nothing of them on the south-west declivity, because^ 

 being there more exposed to the action of the atmosphere, the sur- 

 face is entirely decomposed, covered with debris, and in a state 

 ready to slip downwards in numerous places.* 



3. Erratic Blocks. — With regard to the dispersion of 

 erratic blocks in plains, the circumstances connected with 

 this question are no less easy of explanation on the gla- 

 cial hypothesis. Mr. Lyell has shewn, that heavy masses 

 of rock may be transported to considerable distances on 

 rafts of ice, and thus the block of mica slate, 8 or 10 tons 

 weight, now resting on the soil of the Pentland hills, may 

 have been brought from the Grampians, as no glacier by the 

 mere expansion of its mass could carry this across 50 or 60 

 miles of low country, and deposit it where it now lies at an 

 elevation of 1100 feet.f There are doubtless many other 

 instances similar to this, for which the glacial theory is not 

 sufficient of itself to account. But if it affords an easy solu- 

 tion to all the ordinary phenomena of erratic blocks, such as 

 the dispersion of the granite of Shap Fell over the face of 

 the surrounding country, as described by Dr. Buckland, it is 

 as much as can be reasonably expected, while the few ex- 

 ceptions that occur may be easily accounted for otherwise. 

 The following are the observations of M. Agassiz on this 

 subject : — 



Dispersion of Erratic Blocks in plains.- — The phenomenon of er- 

 ratic blocks and polished rocks is not limited to the chief groups 

 of mountains, but is seen extending over the whole surface of the 

 country where it presents itself ; with this difference, that in the 

 lower regions it assumes peculiar characters, different from those 

 which I have described as belonging to mountains and their valleys. 



Just as the erratic phenomenon is localized in the vicinity and in 

 the interior of mountains, so does it exhibit uniform characters in the 

 low country and in flat regions, covering vast tracts whose limits 



* Edinb. New Phil. Journal, No. 58, p. 286. 



t Vide Address of Prof. Hitchcock to a meeting of American Geologists, Edinburgh New 

 Philosophical Journal, 1842, p 79. 



