2^^o. 1.] PEALE ON JUEA-TEIAS OF IDAHO AND WYOMING. 123 



equivalent of King's Koipato Group, whicli is below tlie Star Peak Group. 

 Tlie localities, however, are too widely separated, and too imperfectly 

 known, as regards the one in my district, to attempt to correlate the 

 two sections more definitely. 



We cannot be too cautious in predicating the age of these beds before 

 a detailed study of the stratigraphy is made, and a careful paleontolog- 

 ical examination of the whole region undertaken and completed. 



The "Meekoceras Beds," when first seen by us, were referred to the 

 Upper Division of the Carboniferous. 



There are numerous circumstances that appear to indicate that in the 

 Jura-Trias, as in other formations, we have at either end of the forma- 

 tion a plane of ijaleontological indefiniteness, as it may be called ; and 

 the probability is that in the immediate vicinity of the hues separating 

 the formations the strata should be considered as transition series. 

 When the deposition of sediments is continuous from one formation to 

 another, it is not so strange, as Dr. White has frequently remarked, that 

 forms of life should continue uninterrui)tedly from one to the other, and 

 that we should find, as in New Mexico,* Triassic plants at the top of the 

 Jura, and as in our district and many other localities Jurassic inverte- 

 brates near the base of the Trias, and as in Nevada palaeozoic types in 

 the Lower Mesozoic. 



In predicating the age of any group of beds, the organic contents of 

 all descrixjtions, vertebrate, invertebrate, and vegetable, should be con- 

 sidered. 



With the accumulation of data, many of the points that are now 

 obscure and doubtful will assume the certainty of facts. 



Until we know more of the rocks under consideration, I shall retain 

 the name Jura-Trias for the formation. 



To recapitulate, the investigations of 1877 in Southeastern Idaho and 

 Western Wyoming indicate — 



1. The lithological separation of the Jura-Trias into three divisions, — 

 the upper and lower calcareous, and the middle mainly silcieous. 



2. The presence of Jiu-assic forms in the upper division, the absence of 

 organic remains in the middle group, and the presence in the lower of 

 Triassic types associated with what have heretofore been regarded as 

 Jurassic species. 



* See Eeport U. S. Geol. Surv. for 1875, 1876, pp. 84-87. 



