282 ECONOMIC GEOLOGY. 



which may have aifected the district under consideration are deter- 

 mined, with an amount of accuracy which it is permissible to give the 

 investigation. 



Should an inspection establish the existence of sedimentary strata, 

 these would probably contain fossils, in which case an examination of 

 these organic remains would be of high importance, whether with the 

 view of discovering the relative geological age of the formation, or for 

 the purpose of comparing them with fossiliferous strata already classi- 

 fied in countries which have been thoroughly surveyed. To render 

 these researches complete, it is also important to institute jjurely 

 physical investigations, such as the determination of the height relative 

 to the sea level, of characteristic elevations, or depressions, either by 

 barometrical measurements or geodesical operations. 



The economic value of the rocks, or minerals contained in the given 

 district, should such be ascertained. Their existence would probably 

 be revealed during the execution of the geological survey, and it 

 would be requisite, either during the progress of that work, or 

 after its completion, before concluding as to their value, to deter- 

 mine as far as possible the nature and extent of such deposits of 

 the useful metals ; the feasibility of extracting them, of preparing 

 them for commerce, or the industrial processes to -which it maybe 

 necessary to subject them, before they became available for the use in 

 the arts or manufactures. 



In connexion with this department, a series of assays and chemical 

 analyses are highly useful to define the per centage of available 

 product contained in metallic ores, or the relative purity of gypsum, 

 marbles, and other non-metallic substances. 



The prospects of obtaining a supply of water by means of Artesian 

 wells are dejiendent on geological conditions ; and as works of this na- 

 ture prosecuted under favorable geological indications have so often 

 been attended with success, a review of the circumstances which render 

 probable the existence of subterraneous accumulations of water under 

 hydrostatic pressure should not be omitted from any comprehensive 

 survey of the mineral resources of a country. 



Another subject which deserves attention is the influence of geolo- 

 gical structure on agriculture, as exercised through the medium of the 

 soils, which are derived either from the decomposition of the subjacent 

 strata, or from detrital matter transported by aqueous agency from 

 more elevated land at a distance. Experience proves that alter a certain 

 time the richest lands become exhausted by cultivation, and only re- 

 acquire their fertilizing properties alter a lengthened period of repose; 

 during which the mineral constituents present in the soil are decom- 

 posed ; thus rendering accessible to vegetative processes those mineral 

 ingredients so essential to the life of phmts. An examination, then, 

 of a district under cultivation in connexion with its geological struc- 

 ture may frequently afford data for conclusions as to the relative 

 duration of its fertility, and if exhausted, of the time necessary for its 

 restoration to the productive condition. 



