=The method of preparing the map.=—To prepare the map, use is made of a geological compass with clinometer attachment, a protractor, and a map base divided into sections like the top of the table, and on the scale of one inch to the foot. Each exposure represented upon the table is “visited” and then located upon the base map in its proper position and attitude. The result is the field map (Fig. 47), which thus represents the facts only, unless there have been uncertainties in the correlation of exposures or in determining the position of the bedding plane. [Illustration: FIG. 48.—Areal geological map constructed from the field map of Fig. 47, with two selected geological sections.] To prepare the areal geological map from the field map, it is first necessary to fix the _boundaries_ which separate formations at the surface; and now perhaps for the first time it is realized how large an element of uncertainty may enter if the exposures were widely separated. It is clear that no two persons will draw these lines in the same positions throughout, though certain portions of them—where the facts are more nearly adequate—may correspond. In Fig. 48 is represented the areal geological map constructed from the field map, with the doubtful area at one side left blank. Some conclusions from this map may now be profitably considered. The complexly folded sandstone formation at the left of the map appears as the oldest member represented, since its area has been cut through by the intrusive granite which does not intrude other formations, and is unconformably overlaid by the limestone and its basal layer of conglomerate. The limestone in turn is unconformably overlaid by the merely tilted sandstone beds at the right of the map. These three sedimentary formations clearly represent decreasing amounts of close folding, from which it is clear that each earlier formation has passed through an episode not shared by that of next younger age. Of the other intrusive rocks, the dike of porphyry is younger than all the other formations, with the possible exception of the upper sandstone. Offsetting of the formations has disclosed the course of a fault, and from its relations to the dikes we may learn that of these the porphyry is younger and the basalt older than the date of the faulting. The dashed lines upon the map (_AB_ and _CD_) have been selected as appropriate lines along which to construct geological sections (Fig. 48, below map), and from these sections the _exposed_ thicknesses of the different formations may be calculated. In one instance only, that of the conglomerate, can we be sure that this exposed thickness measures the entire formation.