STEATA OF WHITE MARL. 149 



distance from the supposed place of eruption at which these deposits have 

 been observed is about 200 miles. 



The resemblance between the volcanic dust described above and verr 

 pure diatomaceous earth is so close that it is difficult to distinguish one 

 from the other by a cursor}' examination ; with the aid of the microscope, 

 however, the diflFerence is at once apparent, as the dust seldom shows even 

 a trace of any organism mingled with it. 



WHITK MAEL. 



At a number of localities in the Lahontan basin there are exposures 

 of white, chalky marl which does not appear in the canon sections we have 

 described, but is exposed locally, mostly on the sides of the basin, and 

 evidently indicates peculiar conditions of the waters in w-hich it was accu- 

 mulated. 



White marls were first observed in the Lahontan basin at the south- 

 ern end of the desert valley, which is connected with the Cai-sou Desert 

 by the narrow pass in which Allen's Springs are situated; during the 

 higher stages of the ancient lake this valley formed a laud-locked bay. 

 That the waters did not extend through the pass at the southern end is 

 shown by a series of ban-ier-bars at about the horizon of the Lahontan 

 beach, which sweep about this portion of the ancient shore in graceful 

 curves. These concentric gravel ridges, or barrier-bars, record a gradual 

 recession of the waters which once filled the valley, and are especially 

 noticeable from the neighboring hills when the slanting, afternoon light 

 brings out their symmetric forms in bold relief Modern drainage has cut 

 a channel through them in a direction at right angles to their general 

 trend, and exposed the following section: 



Feef. 



Well- worn gravel, formhig barrier bars 15 to 25 



Fine sand, cross-bedded 6 to 10 



Finely laminated, white, chalky marl 



Gravel, well rounded, ferruginous Ito 2 



Fine sand ; to bottom of exposure Ito 2 



The marl at this locality is by aneroid measurement 175 feet below the 

 Lahontan beach, and may be traced for a hundred vards or more along the 

 sides of the arroyo. At both its lakeward and shoreward margin it becomes 



