456 CARNEGIE INSTITUTION OF WASHINGTON. 



end and solidly silicified, or with only minute residual carbon, at the 

 other, while trunks from deep shafts just over the coal at Aladdin, 

 Wyoming, show roughly cylindrical, partly carbonaceous zones with 

 more complete silicification, both interiorly and exteriorly. Un- 

 doubtedly such features deserve the attention of chemists and petrog- 

 raphers. 



Petrifaction as seen in the great sauropod skeletons also presents 

 features of sharp interest where the bones are partly to wholly silici- 

 fied, the condition found at the old dinosaur quarries to the east of 

 the Freeze-Outs, and 12 miles north of Medicine Bow. Examination 

 of the polished surfaces, which are of great beauty of pattern, and of 

 the histologically clear sections, shows the bones to be in some in- 

 stances solidly silicified throughout. But there is certainly in places 

 some remaining calcium. The lamellar net is quite black, as if re- 

 taining carbon, and uniformly surrounded by a brownish outer layer. 

 The cancelli are filled with milky to bluish translucent quartz con- 

 taining many inclusions. Just outside the bounding brown band is 

 a less-defined zone of globules with the appearance of ordinary hyaline 

 quartz, but without the nucleation of the spherules so characteristic 

 of hyalite. The interspaces are often filled by amorphous quartz, but 

 banded chalcedony is a frequent feature. In or about the hyaline 

 zone are many small pyrite-crystal inclusions; those of largest size 

 are isolated 0.1 mm. pyrite cubes of full luster. Pyritohedrons may 

 be present and hexagonal tabulate forms may be iron carbonate (?). 

 There are also flocks of minute doubly terminated quartz-crystal 

 inclusions, all oriented in the same direction over considerable areas. 

 It is easy to view this silicification as secondary, but the replacement 

 of the calcium phosphates likely occurred at the time the carbon of 

 the cycads and conifer logs was displaced by silica. 



EVIDENCE OF ARIDITY IN TPIE LAKOTA. 



Where the Lakota ( = Cloverly) outcrops as the easterly extension 

 of the Como Bluff about 16 miles easterly from Medicine Bow there 

 occurs a remarkable series of tufas, often taking a globular form 

 several feet in diameter. These are accompanied bj^ quartz pebbles 

 often smoothed to polished, and by very limy sands. And, as else- 

 where noted, remarkable testimony as to the course of events dur- 

 ing tufa accretion is afforded by the fact that the polished pebbles 

 may be cleanly embedded far within the subradiate structure of the 

 globular tufas. Evidently the pebbles like the well-known ''drei- 

 kanter" type, belong to desert surroundings, just as the tufas are 

 best explained as depositions along the shores of salines or rece- 

 dent lakes. Just where precisely similar conditions may be found 

 to-day is not yet determined, although a very near approach is seen 

 in the tufas of the Lake Lahontan shore-lines. But some variation 



