414 



NON-MARINE FOSSIL MOLLUSCA. 



Subdivisions of geological time. 



No special explanation of the terms or names used in this table, with 

 perhaps the exception of Laramie, is deemed necessary, because they are 

 to be found in ail text-books of geology now in use; but some supple- 

 mentary explanation of the terms that, in connection with those which 

 have become so commonly known, have been used by the various geolo- 

 gists who have made original investigations in Western North America is 

 necessary to enable the general reader to understand clearly the use of 

 them that is made in the following pages. 



The term Laramie Group, although it is unknown except in the geology 

 of Western North America, will be mentioned oftener perhaps in this 

 article than the name of any other formation ; because a greater number 

 of the species herein mentioned come from that group of strata than from 

 any other. This name is applied to a large and very important formation 

 which, in the western part of the continent, conies between the well- 

 recognized marine Cretaceous strata below, and the equally well-recog- 

 nized Tertiary strata above. Geologists are not agreed as to whether 

 this great group should be referred to the Tertiary or Cretaceous period, 

 some contending for the former and some for the latter reference. The 

 truth appears to lie between the two opinions, and I have elsewhere 

 presented reasons for regarding this group as occupying a transitional 

 position between the Cretaceous and Tertiary.* Neither the Lar- 

 amie Group nor any true geological equivalent of it is at present known 

 anywhere except in Western North America. It there occupies or is 

 found at various localities within a large region, the present known limits 

 of which may be roughly stated as extending from Northern New Mex- 

 ico on the south to the British possessions on the north, and from the 

 vicinity of Great Salt Lake on the west to a present known distance out 

 upon the Great Plains of more than 200 miles from the eastern base of 

 tlie Rocky Mountains. It has been traced within tlie western boundary 

 of both Kansas and Nebraska. 



In the course of the earlier geological investigations which were made 

 in the west, the strata of this great group, which represents a distinct 



'An. Rep. U. S. Geol. Snr. Ter. for 1877, pp. 259-265. Ib. for 1878, pai t I, pp. 51, 52. 



