1914] Buwalda: Tertiary Mammal Beds in West-Central Nevada 345 



scopically for diatoms, but, barring one or two doubtful cases, none 

 were found. The total thickness of the shales appears to be consid- 

 erably greater in the middle of the valleys than along the valley 

 borders. 



Several types of limestone occur. All are usually quite impure. 

 A stratified stony variety is by far the most common. Of less fre- 

 quent occurrence are oolitic limestones, lithoid tufa domes, and thino- 

 litic and dendritic varieties of tufa. 



The stratified stony limestone makes up a notable part of the 

 lacustral series, although it never exceeds a few feet in thickness at 

 any one horizon. These rocks usually occur as strata a few inches 

 to a few feet thick, interstratified with shales and sandstones, and 

 persisting horizontally for considerable distances. The limestones are 

 hard, compact, and heavy. They are generally white, light gray or 

 yellowish brown, and often contain remains of fresh-water molluscs. 

 Lamination or banding is rarely shown within the limestones. They 

 are visually very impure, probably containing no small amount of 

 magnesium carbonate. 



The oolites are usually of grayish brown color, loosely cemented, 

 and contain a generous admixture of sandy materials. They are 

 never more than a few feet in thickness, usually only a few inches ; 

 the individual spherules are usually one-eighth inch or less in diameter. 



Lithoid tufa domes, quite similar to those described by Russell 3 

 as occurring in the deposits of the Pleistocene Lake Lahontan, are 

 found at several localities in lone and Stewart valleys. They may 

 be seen standing upright or, where they have overturned, lying hori- 

 zontally on the lacustral strata. These calcareous masses have been 

 entombed since Miocene time and are now exposed, through removal 

 of the enclosing lacustral sediments by erosion. Like the domes de- 

 scribed by Russell, they have a structure concentric to a six to twelve- 

 inch tubelike opening in the center, from which several smaller tubes 

 often branch. The largest domes noted were about fifteen feet in 

 height and of like diameter, and contain many tons of tufa. They 

 consist, however, of but one kind of tufa, the gray or brown lithoid 

 or stony variety, as contrasted with the three different layers — lithoid, 

 thinolitic, and dendritic tufas — reported by Russell in the Lahontan 

 domes. Many small, pipelike white tufa deposits a few inches in 

 length occur in the sandstones ; they may represent the embryonic 

 stages of tufa-dome construction. Calcareous tufa domes similar to 



5 Russell, I. C, Lake Lahontan, U. S. Geol. Surv. Mon., no. 11, p. 207, 1885. 



