4. _Imbedded minerals.—Independent of the grains of quartz and scales of mica irregularly distributed through it, we find it containing large imbedded masses of quartz, of hornblende, and crystals of chiastolite.
5. Subordinate Beds.—It contains a greater variety and number of foreign beds than gneiss or mica-slate ; and of these some are nearly peculiar to it, and charac terise the whole formation. We shall first mention those which arc common to gneiss and mica-slate, as well as clay-slate, and then those that are peculiar to clay-slate.
(I.) Rocks that occur in gneiss, mica-slate, and clay slate. I. Limestone. 2. Hornblende-rock. 3. Primi tive Greenstone. 4. Hornblende-slate. 5. Porphyry. 6. Quartz. 7. ?ictynolite.
(2.) Rocks peculiar to the clay slate formation, or which occur very frequently in it. 1. Whet-slate. It occurs in beds, in Saxony, Bavaria, Silesia, Stiria, and other countries. 2. Roof-slate. This is but a variety of clay-slate, distinguished by its bluish or ash-grey co lour ; its straight slaty fracture ; its splitting into large tables, and its being- nearly pure and unmixed. It sel dom or never forms whole mountains, but occurs usually in single thick beds with other kinds of clay-slate. 3. Chlorite-slate. This usually follows the preceding. It forms whole beds, and includes garnets, crystallized magnetic iron-stone, iron-pyrites, common schorl, tour maline, and quartz. 4. Thlc-slate. This is usually the next in the order of succession. 5. .ellum-slate. It oc curs in considerable beds in clay-slate ; and the two sub-species, the common and shining, alternate with each other. It contains a portion of carbon, and also iron-pyrites. 6. Drawing-slate. It occurs usually in the vicinity of alum-slate, and is very nearly allied to it. It contains more carbon than alum-slate, but less iron pyrites. 7. Potstone occurs in considerable beds. 8. Flinty-slate occurs in considerable beds in this great formation. 9. Lydian-stone occurs in beds and imbed bed masses.
6. Formations.—This rock occurs along with mica slate, :inn sometimes in beds in gneiss, and even in granite.
7. Form of motortains.—It sometimes forms whole ineuntains, and even chains of mountains. Its moun tains have usually a gentle acclivity ; and its cliffs are not so steep and rough as those of mica-slate or gneiss.
It is more favourable to vegetation than any of the rocks we have hitherto described ; and it is observed that the quantity of vegetation increases from granite to clay slate ; and this appears to depend, not so much on the lower level of the outgoings of its strata, as on the na ture of the rock itself.
We can thus observe a gradual change in the shape of mountains, also of their cliffs and valleys, from gra nite to clay-slate; and these differences are so striking and characteristic, that a long experienced eye can, at a glance from the summit of a mountain, point out with considerable certainty the different formations of which a country is composed. Landscape-painters, by con founding together all these differences, or by combining them irregularly, fail not only in accuracy, but in giving their work that appearance which shows, at first glance, that it is not only a copy of nature, but a copy by one who has formed a distinct conception of the most gene ral and particular features of the inequalities observable on the surface of the earth. Some affect to maintain, that the grand features of mountains and plains are dif ferent in different zones. Thus, that in the torrid zone, for example, the shape, cliffs, and other appearances in mountains, arc different from those in the temperate zone. This, however, is a mistake ; for the same for mation in all countries presents similar external charac ters ; and as the great formations are universal, no such differences can exist. It is true, that the blue colour of the heaven, its degree of illumination, the appearance of distant mountain-vapour, the shape of animals, the luxuriance of vegetables, combined with the features of mountains, will form a particular character for each climate ; but still the aspect of the rocks of the same formation, in whatever country they occur, will he the same. Thus cliffs of granite and mica-slate have the same appearance in India and Siberia as in Scotland ; and the valleys of the Urals do not differ in shape and other features from those formed by similar rocks in this neighbourhood.