SECTIONS EXPOSED IN TRUCKEE OAS^ON. 135 



coated with a continuation of the same tufa sheet, the upper limit of which 

 is about 200 feet below the highest of the ancient water-lines engraved 

 on the sides of the valley. The tufa interstratified with the upper clays 

 almost invariably starts from small nuclei, and, forming dendritic branches, 

 spreads out above into dome or mushroom-shaped growths ; in some instan- 

 ces the tufa is prolonged downward below the general level of the stratum 

 to which it belongs, and forms irregular vase-shaped masses below the con- 

 tinuous tufa layei'. Immediately below the tufa, and sometimes adhering 

 to it, are great quantities of Cypris and gasteropod shells, and occasionally 

 bones of fishes, indicating that the waters from which the calcium carbonate 

 forming the tufa was precipitated were far from being concentrated sahne 

 solutions. 



Throughout the section, the contact of the medial gravels with both the 

 underlying and the overlying clays is unconformable, owing, in each case, to 

 the erosion of the lower member, as is well shown in Figs. A, (_% and D, 

 Plate XXVI, which are accurate sketches of observed exposures, and illus- 

 trate the filling of channels, formed principally by erosion, with current- 

 bedded gravel. 



The lacustral clays forming the lower portion of the section are, in 

 places, exposed to the depth of 100 feet, but what their total thickness may 

 be it is not possible to determine from the present exposures. When exam- 

 ined at some distance from the shore of the basin, they exhibit little varia- 

 tion, and are normally finely laminated, marly clays An exception is found, 

 however, a short distance above the Agency Bridge, on the east side of the 

 river, where a rounded boulder of hard volcanic rock from 2J to 3 feet in 

 diameter occurs several feet below the top of the lower clays. This is a 

 much larger block than any seen in the medial gravels, and evidently must 

 have been floated to its present position, probably through the agency of 

 ice. Although rounded and worn it did not exhibit striations or planed sur- 

 faces, and gave no proof that it had ever been subject to glacial action. 



In places, the lower clays exhibit contortions which in some instances 

 can only be accounted for by a movement of the beds since their deposition, 

 caused apparently to the weight of the superimposed masses of gravel and 

 clay. In other exposures the contortions and convolutions of the laminated 



