160 GEOLOGICAL HISTORY OF LAKE LAHONTA^iT. 



incline are usually composed of water-worn gravel, but instances are not 

 rare in the Lahontan basin of fine clays and marls that were formed in 

 even layers inclined at an angle of from 10 to 20 degrees. 



CONTORTED STRATA. 



The folded and contorted appearance presented by many sedimentary 

 beds may originate in two ways; either they were deposited in a horizontal 

 position and subsequently disturbed, or they were laid down in agitated 

 waters in the contorted forms we now find. The Lahontan sediments 

 afford illustrations of each of these modes of origin. 



Examples of contortion and deformation in the lower lacustral clays, 

 obviously due to motion produced by the weight of tlie superimposed de- 

 posits, were observed at many localities. In the Truckee Canon, disturb- 

 ances of this nature occur, as shown in the lower portion of the illustration 

 formino- Plate XXY. At the left of the section the stratum of marly clay has 

 been broken in an irregular manner and one part thrust over the other; at 

 the right, in the same section, the strata are crumpled and folded in such a 

 marmer as to form anticlinals and synclinals in miniature. Other illustra- 

 tions of similar disturbances may be seen in the section exposed along the 

 Humboldt, Truckee, and Walker rivers. At the top of the section shown 

 on Plate XXV, but weathered back so as not to appear in the drawing, 

 there is a deposit of fine yellowish sand in contorted strata resting on the 

 upper clays. This deposit contains crystals and rosette-like masses of 

 selenite, and is evidently water-laid — not seolian as perhaps might be fan- 

 cied — and from its position at the top of the section could never have been 

 subjected to pressure or mechanical disturbance. The contortions and fold- 

 ino-s of the thin sheets of sand composing this deposit are rendered espe- 

 cially distinct, when seen in section, by the presence of iron-stained lines 

 and bands, which indicate a character' of contortion that can only be ex- 

 plained by assuming that the beds were deposited in the irregular forms 

 they now present. Similar contorted beds were observed in the Quaternary 

 strata of the Mono Lake basin, California, in a bed of sand and pebbles 18 



