Area/WorloftJn /". X. (;e<>h>(j!c(d Surmj. — McGee. 379 
represented by hachures disposed either irregularly or in such a 
manner as to indicate structure. The superficial deposits are 
represented by round figures in such a manner that they may be 
mapped in their nc^rmal relation, overlying the older rocks, on 
the sheets showing the underlying formations. The general sys- 
tem provides for the representation of the geolog}' on the topo- 
graphical maps. The atlas sheets are colored in manuscript b}' 
the geologists in the field and the geological symliols are after- 
ward engraved on zinc. In order to make these sheets available for 
all uses, provision has been made for printing each sheet in portfolio 
form, supplemented b}' as many different imi)ressions of the same 
map as may be required. Thus the portfolio will usually include a 
topographic sheet without geological symljols; a geological sheet 
showing only the age-groups and formations: a structure sheet in 
which sections drawn to scale are printed on a sheet showing the 
groups and formation boundaries; sometimes a sheet of columnar 
sections showing the structure in greater detail ; in some cases a 
sheet showing the superficial deposits only; and when the occasion 
■requires, a sheet of mineral resources, showing the location of mines, 
quarries, coke ovens, smelters and furnaces as well as mineral areas. 
These geological surv^eys consume much time. Moreover, a 
variety of circumstances have combined to delay the completion 
of the surveys except in special districts, such as the Lake 
Superior iron region, the (juicksilver and gold regions of Cali- 
fornia, the phosphate l)elt of Florida, the Eureka and Virginia 
City districts in Nevada, and some mining areas in Colorado. 
Final geological surveys of greater or less extent have been 
executed in 32 states and territories. These surveys cover an 
area of 117,000 square miles, and are in part represented on 100 
regular atlas sheets and a large number of special maps. 
The cost of the topographical surveys has varied with the scale 
and other conditions from less than $1 to over $5 per sciuare 
mile. The average cost of the survey', including drawing, has 
been $3 per square mile on the one-mile- to-thc-inch scale and the 
total cost since the first has been about $4 per s(piare mile. The 
cost of the geological survey has varied between much wider limits. 
In fairly representative districts the cost has averaged 15 to $(» 
per square mile. Th(^ average cost from the beginning has aver- 
aged $8 per sipuire mile, l)ut this cost includes preliminary ex- 
penditure on instruments, ])ooks, laboratories, etc. 
