Grey may be said to be the prevailing colour among rocks, especially of the older geological periods. In simple rocks like limestones it is often produced by the intermingling of minute particles of clay, sand, or ironoxide, or of amorphous carbonate of lime with the paler crystalline calcite of the comminuted organisms. Pure crystalline limestone is naturally snow-white, as in Carrara marble. In compound rocks the prevailing grey hues depend on the mixture of a white mineral, usually a felspar, with one or more dark minerals like magnetite, hornblende, or augite, the lightness or darkness of the hue depending upon the relative proportions of the constituents. Should the felspar be coloured by iron, a pinkish hue may be given to the grey; or if the dark magnesian silicates have been altered into some of their hydrous representatives, the grey becomes more or less distinctly green. The old "greenstones" owe their distinctive hue to this source.

V. Smell.-Clay-rocks may be recognised by the peculiar earthy odour they give out when breathed upon. Crystalline felspar rocks when breathed upon often yield this smell. Some rocks, especially limestones containing animal matter or decomposing iron sulphides, yield a fetid or rotten-egg smell when freshly broken.

VI. Feel. A few rocks are characterised by a peculiar feeling to the touch. This is chiefly shown by the hydrous magnesian silicates, talc, chlorite, serpentine, &c. (also by some micaceous schists), which have a greasy or soapy feel. In large tracts of country formed of chlorite-schist, margarodite-schist, or serpentine rock, the stones have everywhere this characteristic. The term "trachyte" was originally applied

F

FIG. 12.-Outlines of mountains formed of stratified or sedimentary rocks. Rocky Mountains. (Hayden's Report of Survey of Western Territories, 1874.)

to certain volcanic rocks distinguished by the harsh, prickly feeling experienced when the finger is passed over their surface. A rock like chalk is said to be meagre to the touch.

VII. Behaviour in Mass.-There are some remarkably characteristic aspects of rocks which cannot be judged of in hand-specimens, any more than the architecture of a building can be told from the nature of the stone employed in its construction. It is as parts of the architecture of the earth's crust that rocks present many of their most typical and individual features. These broader and larger characters show themselves in the outline of every hill and mountain. As illustrations we may take the two contrasted groups of the stratified fragmental and amorphous crystalline rocks. Even from a distance the difference between these rocks makes itself felt in the striking distinctions so often visible in the form of mountains. Thus in Fig. 12 it will be noticed that two prominent sets of lines can be traced all along the crests and declivities—the horizontal lines of the bedding and the vertical lines of the joints. The rocks are cut into huge blocks in the process of denudation, and these blocks are further channelled and chiselled along the dominant divisional lines. With this rectilinear style of architecture compare that of a mass of granite, one of the amorphous crystalline rocks. parallel systems of lines here catch the eye. The crests are splintered, indeed, along the joints, and these divisional lines may be traced by a practised eye down many of the cliffs and steep declivities of granite, but they never show the definiteness, regularity, and alternation of prominent and retiring bands so typical of

FIG 13.-Outlines of a mountain formed of crystalline rock. Rocky Mountains. (Hayden's Report for 1874.)

stratified rocks. The general lines of the mountain are graceful curves rising more and more towards the summits till they often become vertical.

The stratified rocks, then, are distinguished by their arrangement into beds, varying according to the nature of the substance, from the finest laminæ up to large masses many yards in thickness. The amorphous crystalline rocks, on the other hand, are marked by the absence of all structure except their joints. The reader will find this subject further illustrated in succeeding chapters; but he will learn more by a little practice in the field than can be easily communicated by books.