THE WHITE TEREACE. 



151 



not easily determined; no fossils were found, and it is not impossible that 

 tlie marls ni-e of much older date than the associated Quaternary beds; 

 possibly they are of Tertiary age. 



The best localities of all for observing the deposits we are considering 

 are about Pyramid Lake. In this basin thej^ frequently appear as a con- 

 spicuous white band along the borders of the valley at an elevation of 320 

 feet above the 18S2 level of the lake, and form a well defined built-terrace 

 which we have named the "White Terrace." Measurements with an engi- 

 neer's level, as well as many observations with an aneroid barometer and hand 

 level, show this terrace to have a nearly uniform height and to be coinci- 

 dent in elevation with the water-line which marks the upper limit of the 

 dendritic tufa. About the Marble Buttes, and at many points along the 

 steep western shore of Pyramid Lake, the White Terrace is well exposed in 

 sheltered ravines, which were coves and bays when the waters occupying 

 tlie valley stood 320 feet higher than at present, but it is wanting on pro- 

 jecting spurs. Northward of Mullen's Gap the terrace becomes more con- 

 tinuous, and when cut by arroyos exliibits the sequence represented in the 

 following section : 



Flu. 24.— Sccliou of ^yIiitc Tcrrute, west side of P^raniiJ Lake, Xevada. 



In some instances the outer border of the terrace has been removed so 

 that the steep lakeward dip of the strata is not always observable. At a 

 number of localities the terrace is from 200 to 400 yards broad, with a plane 

 or slightly concave upper surface which usually slopes gently lakeward ; 

 the outer scarp is steep and at times 30 or 40 feet high, but the deposit di- 

 minishes rapidly in thickness towards the shoreward margin. The marly 

 beds are usually underlain by alluvium, as shown in the figure, and are 



