Ch. 27] SEARCH FOR EXTENSIONS OF KNOWN DEPOSITS 495 



occur at one or more definite stratigraphic horizons, and this facilitates 

 the search for them to some extent (examples include phosphate de- 

 posits of the type present in the Pliocene Bone Valley formation in 

 Florida, and manganese deposits such as those in the Pliocene (?) 

 Muddy Creek formation in the Lake Mead region and in the Jurassic 

 (?) Franciscan formation in California). Exploration for these de- 

 posits begins with the preparation of a geologic map. Additional de- 

 posits may be discovered in the course of mapping simply by careful 

 examination of available exposures and float. The same result is 

 sometimes achieved without mapping if the geologist disciplines him- 

 self to study the ground with the same thoroughness as when he maps 

 it (provided, of course, that the structure is simple enough that he 

 can trace the. favorable horizons without having to solve complex 

 structural problems). If mapping or ground search does not directly 

 provide the desired results, the favorable horizons are tested at depth 

 by drilling or other means of physical exploration. In most places, 

 such drilling is done on a grid — a sound plan to follow where the factors 

 controlling the distribution of the deposit are not known. In these 

 circumstances, the geologist pays his way by providing the engineer 

 with information on the structure and lithology so that the favorable 

 horizons may be tested at the lowest possible cost. From a knowledge 

 of the habit and shape of the deposits elsewhere in the district, he may 

 be able to help the engineer "drill out" any deposits discovered, and, 

 of course, the geologist is chiefly responsible for the interpretation of 

 the results obtained. 



Other sedimentary deposits of irregular distribution occur in surficial 

 materials whose distribution is related to the physiography. Examples 

 include both ancient and modern beach and alluvial placers, peat de- 

 posits, salines in the playas of the Southwest, clays of glacial origin, 

 and, of course, most sand and gravel deposits. These deposits are 

 extremely important economically, not only because many of the 

 materials have a relatively high unit value (such as the placers and 

 salines) but also because many such deposits are amenable to mining 

 at an extremely low cost. 



The search for surficial deposits is so simplified by their relation to 

 the physiography that prospecting is often done without the aid of a 

 map. Use of a topographic and geologic map, however, may make the 

 difference between a successful, inexpensive search and an unsuccessful, 

 expensive one. This is especially true if the map shows the areal dis- 

 tribution of various types of surficial deposits (kames, eskers, glacial 

 outwash, flood plain, terraces, etc.) as well as local variations in their 

 composition and texture. Knowledge of the location of the lodes from 



