NORTH AMERICAN GEOLOGY. 197 



its former level it appears to liave been highly charged with saline mat- 

 ter, whicli was deposited as a crystalline tufa, of which thiuolite is the 

 pseudomorph. This stage was probably closed by a rise of the lake and 

 consequent dilution, and the dentritic tufa was deposited, overlapping 

 the thinolitic and extending up against the lithoid. The rise continued 

 and the lake finally attained its highest level, covering an area of over 

 8,000 square miles, with a depth at greatest of 886 feet. It then carved 

 the Lahontan terrace, after which it evaporated away, jirobably to com- 

 plete dessication, forming terraces at different altitudes and depositing 

 a thin coat of tufa. The present lakes are of recent origin. 



25. Volcanic dnst deposits in the West. — In an examination of some 

 Pliocene sandstones, collected by Peale in Montana and Idaho, G. P. 

 Merrill* discovered that they are principally composed of particles of 

 volcanic glass and other finely fragmental products of vulcanism. It was 

 soon after found that the sand adhering to fossil bones in the Niobrara, 

 Loup Fork, and Sweetwater regions was of similar composition, and 

 other sandstones of the same character were received from Arizona, 

 Colorado, and from Norton and Phillips County, Kansas, showing the 

 wide distribution of deposits of this kind in the West. 



26. Peale, in a letter to Science, describes Montana deposits similar 

 to the Loup Fork beds. The volcanic dust in both instances appears 

 to have been ejected high into the air from some vent and to have fallen 

 directly into the lake, as the fineness of the particles bears no relation 

 to the proximity of the ancient shore.f 



27. Todd announces the discovery of beds of volcanic dust in Ne- 

 braska, where they are associated with what appear to be ice-lloe beds 

 of drift. He considers them to have been deposited during the Quater- 

 nary, and probably at one stage of King's Lake Cheyeune.| 



SOUTHERN TERTIARY. 



The controversy on the order of succession of the Tertiary beds of the 

 Gulf States is still kept up by Meyer, who persists in his theory that the 

 Grand Gulf group is the base of the series, in opposition to many ob- 

 servers. 



28. Hilgard, in a letter to Science, severely criticises Meyer for neglect- 

 ing the previous literature and persistently ignoring well-known facts 

 disproving his theory. He declares, for instance, that the statement, 

 liointedlj" made and verified innumerable times, that "the sandstone of 

 the Grand Gulf group is found overlying the Vicksburg strata gener- 

 ally along the southern line of the Vicksburg grou[)" is entirely disre- 

 garded, although Meyer was often near the outcrops at which this could 

 be plainly seen ; and again, that in referring to the re appearance of the 

 Jackson shell bed at one point on the Chickasaw Eiver south of the 



* Am. Jour. Sci., in, 33, pp. 199-204. 

 t Science, vol. 7, p. 1G3-1G5. 

 \ Hid, p. 373. 



