10 
FLORIDA STATE GEOLOGICAL SURVEY. 
their chemical as by their physical properties. To properly test 
a clay it is therefore necessary to install the testing machinery. 
Effective clay testing machinery will require for installation more 
space than is now available in the Survey rooms. 
THE PREPARATION OE A DETAILED TOPOGRAPHIC MAP OE FLORIDA. 
While a general topographic map of Florida with contour 
lines at 50 foot intervals of elevation has been issued, as already 
stated, there is a constant demand for detailed topographic maps 
on a scale of about one inch to the mile and with contour lines at 
10 foot intervals of elevation. Topographic maps are usually 
made in atlas sheets covering unit areas bounded by parallels and 
meridians. The unit adopted by the United States Geological 
Survey in topographic mapping designated as the quadrangle, 
includes when made on the scale of about one inch to the mile an 
area of 15' of latitude by 15' of longitude. A separate atlas sheet 
is issued for each unit area and when completed the maps so 
issued make up a complete map for the State as a whole. The 
maps thus made show the land area in relief by means of contour 
lines. In this way all hills, valleys, stream. channels, sinks, de¬ 
pressions and all changes in elevation are indicated. The actual 
elevation above sea, based on exact levels, are also shown by 
means of figures printed on the contour lines. Each contour 
passes through points which have the same altitude. One who 
follows the contour on the ground will go neither up hill nor 
down hill but on a level. By the use of contours the shapes of 
the plains, hills and valleys as well as their elevations are shown. 
The line of the sea coast itself is a contour line, the datum or 
zero of elevation being mean sea level. The contour line at, say, 
20 feet above sea level is a line that would be the sea coast if the 
sea were to rise or the land to sink 20 feet. Such a line runs 
back up the valleys and forward around the points of hills and 
spurs. On a gentle slope this contour line is far from the present 
coast line, while on a steep slope it is near it. Thus a succession 
of these contour lines far apart on the map indicates a gentle 
slope; if close together a steep slope; and if the contours run 
together in one line, as if each were vertically under the one 
