128 



GEOLOGICAL niSTOllY OF LAKE LAHO^'TAN. 



identical conditions; they are both evenly laminated, fine-grained, drab- 

 colored clays, that are usually marly and saline, and frequently exhibit a 

 well-marked jointed structure. An analysis of a typical example of each, 

 as reported by Dr. T. M. Cliatard, shows that they do not differ more widely 

 in composition than might be expected in two samples taken at different 

 points in the same s.tratum. 



Constituents. 



Loss by ignition (w.ater) 



SiliciX (SiOj) 



Alumina and ferrous oxide (AI2O3, and Fe203) 



Lime (CaO) 



Magnesia (MgO) 



Potassa (K2O) 



Soda (NazO) 



Total 



The upper clays differ from the lower, however, in the fact that at 

 some localities they include interstratified beds of homogeneous, white, 

 pumiceous dust, forming even layers from a fraction of an inch to several 

 feet in thickness; and also a deposit of tufa in peculiar mushroom-shaped 

 forms. The layer of fine marly clays on which the tufa stratum rests fre- 

 quently teems with Ci/pris cases, and sometimes contains the shells of 

 FomjiJioli/x effusain immense numbers; above the tufa the shells oi Anodonta 

 nuttaU'uma are frequently abundant. In the lower clays the relics of mol- 

 luscan life are comparatively rare; and, so far as has been observed, they 

 contain no deposits of volcanic dust; this, however, may be considered as 

 an accidental circumstance dependent on the periods of eruption of distant 

 volcanoes. The layer of tufa in the upper clays is a widely spread deposit 

 indicating chemical conditions that so far as is known — the entire thick- 

 ness of the lower clays not being exposed — did not occur previous to the 

 formation of the medial gravels; although lenticular masses and thin sheets 

 of tufa of a somewhat similar nature are not uncommon in the lower por- 

 tion of the Lahontan section. 



The medial gravels are in many places plainly divisible into two 

 portions; the lower, composed of clean, well-worn, current-bedded sand 

 and gravel, has all the structural characteristics of stream-bed and shore 



