The earliest geological maps represented the variations in the surface soils, or at most the general lithological characters of the rocks which by their decay furnished the materials for the soils. We have seen that the first chronological map was due to William Smith, and most subsequent English geological maps have been based upon his map of the strata of England and Wales. The order of succession of the strata is represented in these maps to some extent by the use of arrows to indicate the direction of dip of the strata, though this is not an unerring guide where strata are reversed, and accordingly the addition of a legend at the side of the map may be looked upon as essential to the correct understanding of the map itself. The legend is usually in the form of a section of a column, the strata being arranged in right order, the oldest at the base and the newest at the summit, the colours by which the strata are indicated being similar to those placed upon the map. Other information besides the mere order of succession of the strata may appear in the legend; thus their relative and actual thicknesses can be indicated if the column is drawn to some definite scale, and a brief description of the lithological characters of the rocks may well be appended to the side of the column. On the actual maps it is customary to exhibit the outcrop of the junctions of all igneous rocks as well as of the sedimentary ones: the nature of the metamorphism which sedimentary rocks have undergone at the contact with igneous ones may be and often is indicated by suitable signs; the position of faults is shown, and often also that of metalliferous veins, the nature of the ore in the latter being further indicated in some suitable manner, as by giving the recognised symbol for the metal; and in many maps an attempt is made to show the variations in dip and strike of the cleavage-planes.