158 CLASSIFICATION OP THE PUBLIC LANDS. 



The study of the rock structure in an area is valuable not only 

 in relation to its water supply, as has been indicated, but also as 

 bearing on the occurrence of oil. The accumulation of oil and gas 

 in the strata is governed by laws which are probably akin to those 

 which direct the movement of water, and a knowledge of the rock 

 structure is therefore, as a rule, of great assistance in the location 

 of an oil well. Some new oil fields have been prospected and opened 

 almost entirely on geologic evidence of this kind. It has also hap- 

 pened that the details of rock structure observed in a field examined 

 merely for coal have been found to have an entirely new applica- 

 tion and significance in subsequent prospecting for oil. In view of 

 this close interrelation of geologic phenomena, therefore, the field 

 geologist carefully records all the facts he observes in the course 

 of his work, even though they appear to have no present bearing 

 on the subject of land classification. 



SCIENTiriC DATA. 



Data of interest at the present time chiefly to the scientist form 

 another important " by-product " of the process of land classifica- 

 tion. It is difficult, however, to select any group of facts as being 

 of strictly scientific value alone, for new discoveries may at any time 

 enlarge their significance and impart to them an unexpected economic 

 importance. Many facts are, moreover, of equal interest to the 

 pure scientist ai;id to the engineer or the layman, so that while a 

 great amount of scientific information has been collected in the work 

 of land classification during the last six years, the purely eccnomjc 

 value of much of it has already warranted the necessary expense. 



Many data concerning the areal extent of different geologic forma- 

 tions have been gathered in the last few years and will be of use 

 in connection with the geologic map of the United States which the 

 Survey is now engaged in constructing. The stratigraphic relations 

 of the formations, their composition, and their fossils constitute the 

 only chronicle of conditions which have, at different times and divers 

 places, existed on the earth. From this record some idea can be 

 gained of the former geography of the country, of the great earth 

 movements such as culminated in the formation of the Eocky Moun- 

 tains, of the climatic conditions which formerly existed, and of the 

 ancient plants and animals. These matters are at present chiefly 

 of philosophic interest, but it has been by the accumulation of such 

 facts in the past that science has been enabled to render valuable 

 :tid to mining and other industries. Furthermore, information of 

 this kind, while perhaps of little present interest to the average lay- 

 man, is of vital importance to the teachers of science in the univer- 

 sities of the country. As a case in point may be mentioned the recent 

 discovery, by a Survey geologist engaged in the classification of coal 



