530 STEVENSON— THE FORMATION OF COAL BEDS. [Nov. i, 



wide. The slope of the talus is 30 degrees near the shore but,, 

 within 300 meters, it falls to 20 degrees. The surface is covered 

 with fine silicious sand, a homogeneous black mud, which also 

 covers the lake bottom throughout nearly the whole extent. The 

 talus must become continuous with the bottom deposit within three 

 fifths of a mile. Martins thinks that the slope of the delta proper 

 is 20 to 23 degrees, but is certain that the deposits are horizontal 

 in most of the area, for Martins found the bottom a level plain. 



The delta of the Rhone, at the head of Lake Geneva, was studied 

 by De la Beche,^'" whose results have been presented summarily 

 by Lyell. The lake is 37 miles long and 2 to 8 miles wide. The 

 depth is more than that of Lake Brienz, varying from 20 to 160 

 fathoms, but only from 120 to 160 along the middle line. The 

 Rhone enters at the head as a turbid stream but is limpid at the 

 outlet. An old Roman town, at the shore 8 centuries ago, is now a 

 mile and a half inland. The older portion of the delta, above that 

 town, extends 5 or 6 miles and is a flat alluvial plain, little above 

 the stream and covered with swamps. The surface of the sub- 

 merged cone sinks very gradually and, at a mile and three quarters,, 

 merges with the bottom of the lake, which is covered with river mud. 

 Fine and coarse materials alternate in the delta deposit. When 

 snows melt on the mountains, the increased flow brings down sand, 

 mud, vegetable matter and driftwood. In 8 centuries there has 

 accumulated a formation, perhaps 600 to 900 feet thick and nearly 

 two miles long, with strata only slightly inclined. Conditions are 

 somewhat difl^erent where a delta is formed by a torrent having 

 great speed and a moderate quantity of water. The depth opposite 

 the torrent of Ripaille is 80 fathoms at half a mile from shore, 

 so that dip of strata in that minor delta must be not less than twice 

 as great as in that of the Rhone, or apparently not far from 10 

 degrees. 



Gilbert's^^^ descriptions and figures of the well-dissected deltas 



''"H. T. De la Beche. Edinb. Phil. Journ., Vol. II., 1820, p. 107; C. 

 Lyell, "Principles of Geology," New York, 1872, Vol. I., pp. 413-415. 



'" G. K. Gilbert, " Lake Bonneville," U. S. Geol. Surv. Monographs, 

 Vol. I., 1890, p. 162. 



