[Footnote 30: It is useful to adopt conventional symbols for the representation of strata of different lithological characters, and so far as possible to adhere to the same kind of symbol for any one kind of deposit. Those which are generally in use, are rough pictorial representations of the characters of the deposits, as shown in Fig. 7. The conglomerate is indicated by circular marks representing cross-sections of the pebbles, a breccia by triangular marks signifying that the fragments are angular and not rounded; a sandstone is indicated by dots to represent the grains of sand; a mud, clay or shale by continuous or broken horizontal lines, which reproduce the appearance of the planes of lamination so frequent in beds of this composition; a limestone is usually marked by the use of regular horizontal lines illustrating the pronounced bedding, with vertical lines at intervals to represent the regular jointing which occurs in so many limestones: the nature of the bedding may be further shown by drawing the lines comparatively far apart when the limestone is a thick-bedded one, nearer together when it is thin-bedded. Igneous rocks are represented by crosses or irregular V-shaped marks, illustrating the absence of stratification and presence of joints. Volcanic ashes are sometimes represented by dots, at other times by signs somewhat similar to those which are used for true igneous rocks. Sedimentary rocks which are composed of more than one kind of material may be further shown by a combination of two symbols, thus the existence of a sandy clay may be shown by means of a combination of horizontal lines and dots, and so with other combinations. The practical geologist should become accustomed to the use of these symbols in his note-book; if used, they will save much writing. These symbols are used in some of the later illustrations to this book.] The horizontal section is one which is in constant use by the practical geologist: the results of the first traverse of a district may be jotted down in his note-book in the form of a horizontal section (with accompanying notes), and the written memoir on the geology of any district composed largely of stratified rocks will almost certainly require illustration by means of these sections. Perhaps nothing more clearly marks the careful observer than the nature of the sections which he makes, and geological literature is too frequently marred by the publication of slovenly sections. A badly drawn section not only offends the eye, it may and frequently does convey inaccurate information. [Illustration: Fig. 8.]