Features on the early geologic maps were illustrated by symbols, ruling, stippling, letters, and numerals. From 1775 to 1851 hand-colored maps on printed bases were the chief medium of illustration. Chromolithography began to supersede hand coloring about 1851, and the present multicolored maps are results of the development of color printing. Hand-colored maps are still made but not published.
About 300 of the 375 maps mentioned were personally examined, but they represent only a small part of the thousands in existence.
Maps that are outstanding because of early date, cartographic technique, important contribution to geology, advances in methodology, or some other special interest are described. Maps for all parts of the world are included in the descriptions, but most of the discussion and description is of maps of Europe and North America.