1881.I Miniature Physical Geology. 711 
repel the striking substance with a vigour equal to that of 
the blow. This may be the secret of all elasticity — this 
forcible preservation of lines of movement under the influence 
of the attraction to which they are due. The elasticity of 
liquids may be due to movements of rotation ; that of solids 
may arise from the forcible preservation of their lines of 
vibrating movement. The more rigidly these are retained, 
the more elastic the solid. But when the particles are not 
held with an attractive vigour sufficient to draw them back 
to the points from which they are driven, the solid becomes 
inelastic. When particles freely assume new lines of move- 
ment, elasticity ceases. 
II. MINIATURE PHYSICAL GEOLOGY. 
By C. Lloyd Morgan, F.G.S., Assoc. R.S.M. 
(Concluded from page 640.) 
VIII. The Cutting Backwards of Rivers. 
8 3 HE ordinary typical river forms and deepens its valley 
^ by a combination of two processes, cutting downwards 
and cutting backwards. It is interesting to note how 
closely the two methods are connected. At first sight, in- 
deed, it would appear that most valleys are almost entirely 
produced by the river cutting downwards. But a little fur- 
ther thought enables us to see that this very aCtion is pro- 
duced by the fall of the river, for if there be no fall the 
stream ceases to flow ; and it is this fall of water that causes 
the cutting backwards. It is convenient, however, to keep 
the term ‘ cutting backwards ’ for the case of a river forming 
its valley or ravine by means of a waterfall. 
The most notable example of this on a large scale is, of 
course, the formation of the Niagara ravine, some 7 miles 
long and about 166 feet in height, which stretches from 
Queenstown to Niagara Falls. The water of the Niagara 
River falls over a ledge of limestone. Below this limestone 
is a shale which — being easily decomposed and readily acfted 
upon by the spray that is continually rising from the Falls, 
aided, I have no doubt, to a great extent by the adtion of 
frost— is rapidly undermined, until huge blocks of the lime- 
