The colored areas upon the geological map are each supposed to indicate where a certain rock type or formation lies immediately below the surface, and this distribution represents the best judgment of the geologist who, after a study of the district, has prepared the map. Unfortunately the conventions in use are such that his observation and his theory have been hopelessly intermingled in the finished product. Armed with the geological map, the student who visits the district finds spread out before him, it may be, a landscape of hill and valley, of green forest and brown farming land, which is as different as may be from the colored puzzle which he holds in his hand. Hidden under the farm vegetation or masked by the woods are scattered outcroppings of rock which have been the basis of the geologist’s judgment in preparing the map. Experience shows that in order to bridge the wide gap between the geology in the landscape and the patches of color upon the map something more than mere examination of the colored sheet is necessary. We shall therefore describe, with the aid of laboratory models, the various stages necessary to the preparation of a geological map, and every student should be advised to follow this by practical study of some small area where rocks are found in outcrop. Though the published _areal geological map_ represents both fact and theory, the map maker retains an unpublished _field map_ or map of observations, upon which the final map has been based. This field map shows the location of each outcrop that has been studied, with a record of the kind of rock and of such observations as strike, dip, and pitch. Our task will therefore be to prepare: (1) a field map; (2) an areal geological map; and (3) some typical geological sections. =Laboratory models for the study of geological maps.=—In order to represent in the laboratory the disposition of rock outcrops in the field, special laboratory tables are prepared with removable covers and with fixed tops, which are divided into squares numbered like the township sections of the national domain (Fig. 47). To represent the rock outcrops, blocks are prepared which may be fixed in any desired position by fitting a pin into a small augur hole bored through the table. The outcrop blocks for the sedimentary rock types are so constructed as to show the strike and dip of the beds. (See Appendix D.) [Illustration: FIG. 47.—Field map prepared from a laboratory table.]