ILLUSTRATIONS OF GEOLOGICAL STRUGTUKE. 161 



inches thick, inclosed between horizontal, evenly-bedded, ripple-marked 

 clays and sand. In this instance the iron-stained lines marking the edges of 

 contorted sheets, form a most intricate pattern when seen in section, and in- 

 close pockets and cells sometimes four or five inches in diameter, that are 

 without openings and packed full of gravel and stones; in some instances the 

 pebbles thus enclosed are an inch or more in diameter and are all well water- 

 worn. The presence of these cells filled with material of a different nature 

 from the contorted sheets of sand, while the strata above and below the 

 contorted layer are of fine sand and clay in even horizontal beds which 

 show no crumpling, is evidently proof that the disturbance causing the 

 irregularities of the deposit took place during the deposition of the strata 

 and cannot be referred to subsequent mechanical movement. The condi- 

 tions under which these contorted sands were accumulated are difficult to 

 determine, but in some instances deposition seems to have taken place in 

 shallow lakes that were greatly disturbed by winds and cuirents. The 

 hypothesis which attributes the contortion of superficial strata to the action 

 of advancing glaciers and grounded icebergs is not here admissible, as the 

 relation of the lakes and glaciers is well known. The action of a moving 

 ice sheet, formed by the freezing of a lake, might perhaps under certain 

 conditions disturb the sediments beneath, and might even transport pebbles 

 from the shore and drop them in ofi'shore deposits; thus forming strata 

 analogous to the exposure observed near Mono Lake. It is impossible, 

 however, to account completely for all the phenomena observed by any of 

 the hypotheses that have been suggested. 



ARCHES OF DEPOSITION. 



The finest example of an arch of deposition that has been observed in 

 the Lahontan s edinients is represented in the section forming Plate XXV, 

 and has already been noticed in describing the exposure to be seen along 

 the Truckee River. This, with scarcely any doubt, is a section of a gravel 

 bar, the top of which was removed previous to the deposition of the 

 superimposed gravels. Similar arches, but less complete, may be seen in 

 other portions of the Truckee section, and occur in greater or less perfec- 

 tion wherever a cross-section of a current-formed embankment or bar is 



MON. XI 11 



