POST-QUATEIIlSrARY DEPOSITS. " 153 



When examined microscopically the marl reveals a great quantity of 

 crystallized and amorphous calcium carbcniate, very similar in appearance 

 to the same substance obtained by precipitation in the laboratory, together 

 with other bodies which appear more clearly when the material is treated 

 with diluted acid. On examining the residue insoluble in acids under the 

 microscope, is found to contain many diatoms, especially in the finer and 

 more flocculent portion of the sediment ; the coarser portion, which subsides 

 most quickly, also contains diatoms, but is mainly composed of crystalline 

 grains and many glassy flakes similar to those composing the volcanic dust 

 described on page 14G. The chemical and microscopical examinations 

 render it evident that the material in question is mainly a chemical precipi- 

 tate, but is also, in part, of mechanical and organic origin. 



It seems probable that the calcium carbonate forming the piincipal por- 

 tion of the marl was precipitated from the waters of Lake Lahontan in a 

 microcrystalline and amorphous form at about the time the dendritic tufa 

 was being accumulated ; and became mingled with the siliceous exuviae of 

 the microscopic organisms that lived in abundance in the lake waters ; it 

 also received some contributions of volcanic and aiolian dust, but, in the 

 main, was free from the products of ordinary stream erosion. The deposit 

 thus formed, when near the shore, was assorted and rearranged by currents 

 so as to form the terrace and embankments we now find. In the deeper 

 portions of the lake the lime precipitated from the waters was mingled with 

 clay and sand and now appears as marly-clay. The abundant precipitation 

 near the shore may also have been due, in part, to the greater abundance of 

 nuclei which tended by their jjresence to induce crystaUization 



^EOLIAN SANDS. 



The accumulations to be described under this head consist mainly of 

 sand that has been blown about by the wind and finally deposited in banks 

 or dunes which sometimes cover large areas. 



The first acquaintance the explorer in the Great Basin usually makes 

 with the material forming these deposits is when it is in motion and fills 

 the air with clouds of dust, sand, and gravel, which are blinding and irritat- 

 ing, especially on account of the alkaline particles which saturate the 



