92 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



tlie river there are large tracts of land which might be classified as 

 flood plains. The material composing these plains is chiefly of clay 

 and sand with a mixture of fine gravel scattered throughout the beds. 

 This material is of a rusty yellow or orange color and is similar in 

 texture to the low lying islands of sand and gravel formed at various 

 places in the river ; there is, however, no clay in the islands. 



The outer margins of the ri^-er as it sweeps around these great bends 

 are composed of beds of light blue rock of the Xa.shville group to the 

 west of Nashville, and of the darker colored Trenton limestones to the 

 east. Both form high embankments varying from 100 to 200 feet iii 

 height. Along the limits of the flood plains where it joins the high 

 land the rock wherever visible shews a beveled cut ofi" face and 

 wherever exposed to the air the action of the stream has be6n 

 gi'eatly supplemented by the action of the atmosphere. The edges 

 are rounded off" and even marked in many places with small channels 

 or waterways for the escape of rain, and the overlying soil has rill 

 marks and rain channels corresponding to the marks in the rock. 



The climatic conditions consisting of temperature and rainfall 

 obtaining in the region occupied by the Central Basin together with 

 the material of which the rock is compo.sed and the general drainage 

 system all tend to facilitate the work of erosion. 



The direct influences of temperature are comparatively simple. 

 Temperature aflects erosion chiefly by its changes. When the range 

 runs so low as to include the freezing point of water, frost contributes 

 its aid to weathering. It is only under conditions in which the 

 changes are great and sudden that rocks are fractured by unequal 

 expansion or contraction. 



The range of temperature within the basin includes the freezing 

 point of water. The highest temperature recorded in Nashville 

 between 1871 and 1887 was 1 0-1 in August, 1874, the next being 

 101 in July, 1881. Although it frequently touched the nineties 

 and once reached as high as 99 it never passed the 100 except on 

 these two occasions. In January, 1884, the lowest recorded tempera- 

 ture, — 10, for the same period was reached. On two other occasions — 

 December, 1876, and February, 1886, the recorded temperatures were 

 — 2 and — 7 respectively. Throughout that period the mean of the 

 highest temperatures was 68.3 and the lowest 50.7 with a mean daily 

 range ot 17.7 and a monthly mean of 47.2. The mean annual tem- 



