XXIV REPORT UNITED STATES GEOLOGICAL SURVEY. 



of life commences at the close of the Cretaceous epoch, and, without any 

 line of separation that can yet be detected, continues on upward until 

 only ljurely fresh-water forms are to be found. Dr. White, an eminent 

 paleontologist and geologist, says that the line must be drawn some- 

 where between the Cretaceous and Tertiary epochs, but that it will be 

 strictly arbitrary, as there is no well-marked physical break to the sum- 

 mit of the Bridger group. 



If, however, a well defined nonconformity is found to exist it will be 

 examined with great care and its proper value given, but up to the pres- 

 ent time the views as stated above are sustained by the facts, so far as 

 the investigations by this survey have extended. 



Excellent progress has been made in the report on the general geol- 

 ogy of the country west of the 94th meridian. It is the intention of the 

 survey to discuss the geology of the portions of the West which have been 

 reported upon either by the parties under my charge or by others. It is 

 believed that there is a remarkable unity in the geological structure of the 

 entire area; that although formations of the same age have received a 

 great variety of local names, they will be so correlated that a single sys- 

 tem of classification will include them all. As an illustration the pliocene 

 lake basins which have received a variety of names, as Loup Fork, 

 North Park, Uintad Group, Brown's Park, Humboldt Group, may all 

 be brought under one generic terra and into one geological horizon. 

 The Cretaceous divisions which are so well marked in the Northwest, 

 and have already received suitable geographical names, can be extended 

 to the Pacific Coast, and all the fragments be brought into one group 

 or another. I had intended to publish several chapters on this subject 

 in this report, but the pressure of other duties prevented their comple- 

 tion in time. 



In order to show the unity more perfectly a map of the country west 

 of the 94th meridian has been prepared, dividing the area into twenty- 

 two rectangles, each of which on a scale of twelve miles to one inch will 

 form a map of the size of the " General Geological Map " in the " Atlas 

 of Colorado." It is believed that in no other way can a systematic idea 

 of the geology as well as the geography be obtained. 



The relation of the topographical surveys to the general system of 

 public land surveys is important, and the economic resources have 

 always been a leading feature in our plans. The following paragraphs 

 from the letter of instructions of the Secretary of the Interior show the 

 importance attached by the department to these features : 



In the prosecution of your surveys you will, when necessary, consult such public land, 

 surveys as have been made under this department, in the field of your work, for the 

 purpose of connecting the established lines with your system of triangulation, and of 

 accurately designating on your maps the i^osition of mineral claims. You will deter- 

 mine as far as possible the boundaries of Territories and Indian reservations, and mark 

 the same by suitable monuments. You will also ascertain the position of all agricul- 

 tural lands, and of such mineral lands as you may discover, by trigonometrical rneas^ 

 urements, placing suitable monuments thereon for the guidance of the surveyors-gen- 

 eral of the several districts which may be explored by you. 



