152 STANTON & KNOWLTON — LARAMIE AND RELATED FORMATIONS. 



of the productive measures or by the bed of massive sandstone usually found 

 immediately below the lowest coal bed." 



The Laramie as thus defined was made to include beds containing 

 marine Cretaceous fossils such as Inoceramus. The logical result of such 

 a definition would be a return to the early usage under which all the 

 workable coals of the west were referred to the " Lignitic," and the base 

 of the Laramie would vary at different localities from the Fort Benton 

 to the top of the Fox Hills. Would not the economic considerations be 

 as well satisfied if these older coal beds are called simply Upper Creta- 

 ceous, reserving the term Laramie for beds later than the Fox Hills ? 



It is admitted that owing to the absence or scarcity of fossils it will 

 sometimes be difficult to locate the exact top of the Fox Hills, and, as 

 in the case of Point of Rocks, localities may be known for years before 

 they are assigned to the proper horizon, but such cases are not unknown 

 in other formations. It should be noted that wherever marine fossils 

 have been found above supposed Laramie coal beds they are of Fox 

 HihVspecies (or Montana species, to use the more general term), and 

 this fact is of more importance in age determination than the occurrence 

 beneath them of "characteristic Laramie species." It is generally ad- 

 mitted that marine invertebrates are more accurate and definite horizon 

 markers than either plants or non-marine invertebrates, because they 

 have a less extended vertical and a broader geographic distribution. We 

 would therefore follow the example of King, Hayden, and many other 

 geologists in placing the base of the Laramie immediately above the 

 highest marine Cretaceous beds of the Rocky Mountain region. We in- 

 clude in the Montana formation or division intercalated non-marine 

 beds that at some localities yield land plants and brackish and fresh 

 water mollusks as well as coal. Whether it will be useful to give local 

 formation names to these coal-bearing beds as has been done with the 

 Belly River series in Canada is a question that may well be left to the 

 field geologist when he takes up their detailed mapping. In Bitter 

 Creek valley Powell's Point of Rocks group would be available after 

 some changes in its boundaries, as it has yielded most of the plants and 

 some of the invertebrates of the lower beds. 



It will be instructive to bring together for comparison with the Laramie 

 all the known non-marine invertebrates and the plants associated with 

 the lower coal beds under consideration. 



NON-MARINE INVERTEBRATES OF THE MONTANA FORMATION. 



It is noteworthy that strictly fresh-water forms are very rare,* being 



* The Belly River beds have yielded a considerable fresh-water fauna, consisting mostly of 

 Laramie species with some that continue on into the Fort Union, but as we have no personal 

 knowledge of the stratigraphy of the Canadian beds their fauna will not be considered in detail. 



