STRUCTURE OF SEDIMENTARY DEPOSITS. 137 



hour a column of boiling water, eighteen feet in diameter and one 

 hundred and fifty feet high. Analogous springs of hot water exist 

 in the Rocky mountains, and in India, as well as in Saint Michael's 

 (Azores), where the sili'cious deposits are found in thin beds, alter- 

 nating with argilla'ceous substances which the same waters bring 

 from the interior of the earth. Organic remains, particularly vege- 

 table, are found in all, some of which have passed into the sili'cious 

 state, while others have disappeared, leaving only their impressions 

 behind. 



33. Structure of sedimentary deposits. Effects of land-falls. 

 If we examine deposits of de'tntus, formed at the foot of mountains 

 by the daily destruction of its rocks, it will be seen their slopes 

 are very variable, the greatest not exceeding an angle of forty-five 

 degrees, and the least being seldom less than twenty degrees ; the 

 variations between these limits are found to be in relation to the 

 size, the form of the fragments, and circumstances of the fall, 

 rather than to the nature of the substances themselves. Hence it 

 is, if, at different successive fallings, there are variations in the form 

 of the fragments and in the circumstances of the fall, there will be 

 an accumulation of deposits, the slopes of which will be succes- 

 sively less, and which, in ravines 



excavated by water, will have _ 

 nearly the arrrangement repre- j| 

 sented, a, b, c, d ? e, (Jig. 210), | 

 ^here each additional deposit is " 

 thicker at its base than at the 

 upper part. It is evident the 

 same thing may take place in 

 stagnant waters ; whence it fol- 

 lows that from the fall of a river . " _ . , c , ... 

 into a lake with steep banks, a F'S- ^- Talus from falhng. 



very considerable ta'lus may be formed, and from different acces- 

 sions or growths, which bring materials of different form and size, 

 deposits similar to those just mentioned may be produced. 



34. Effects of transport. If in some places, even under water, 

 beds may be deposited at an inclination of from twenty to forty-five 

 degrees, it must not be inferred that the same is true of extensive 

 deposits, where running waters, if unimpeded, may force the debris 

 in every direction. Here the inclination of the ta'lus is much less ; 

 they never attain even the minimum angle of slopes formed of 

 fallen matter, and never reach even ten or twelve degrees, only in 

 exceptional cases of very rapid torrents, or rather of true cascades, 

 at the place where they fall into a transverse valley, and where 

 there is as much matter tumbled down as transported. The beds 

 of the most rapid rivers are much less inclined, and the successive 



33. What is the structure of deposits from land-falls ? 



34. \A the angle or slope of a ta'lus always the same ? 



12* 



