by pressure, solid conglomerates were formed, consisting of pebbles cemented together by the sand or clay, and interstratified with beds of sand or clay. These deposits have taken place in all periods, but have never attained any great extent at one time or place. Hence the conglomerates play a very subordinate part in the sedimentary formations. In Germany, there is properly speaking but one very characteristic conglomerate formation, which is that of the Rothliegende. The Nagelflue of the Molasse formation is quite subordinate to the sandstone, which is the predominant rock of that formation.

10. Coal Formations.-The greater part of these formations originally consisted of peat, or vegetable materials washed together; usually sand and clay were likewise contained in the deposit, and sometimes hydrated oxide of iron or protocarbonate of iron. These deposits, in the course of time, with pressure, were formed into strata of alternate sandstone (usually grey) and slate-clay or shale; and between these strata, beds of brown or black coal or anthracite and clay-ironstone were formed, subordinate, however, in extent and thickness to the sandstone, slate, and shale. Coarse conglomerates, marl, or limestones very rarely occur in these formations.

The Carboniferous period and the Tertiary period furnish the most characteristic examples of these formations; but the carbonaceous deposits of other periods are associated with similar rocks, and are so like the genuine coal formations that, petrographically, they are hardly to be distinguished from them.

11. Rock-salt Formations.-Rock-salt is always accompanied by gypsum and anhydrite, and it likewise usually occurs in combination with argillaceous deposits. The rocks of this group are usually imbedded in limestone or dolomite, as in the Muschelkalk of Germany, or in sandstone as in Galicia and Transylvania. In all periods these local deposits appear to have taken place, but

the special conditions and causes of their origin are not yet known with certainty.

If we turn to the rocks which we consider to be of metamorphic origin, the crystalline schists, we find alternating beds analogous to those of the sedimentary rocks, but in an altered state. We, kowever, seldom or never find rock-salt or gypsum, a circumstance which may explained by the great solubility of those rocks.

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The crystalline schist formations may be best described by naming the principal rocks of each. Thus we have :12. Argillaceous Mica-schist Formations, with subordinate beds of quartz-schist, lydian stone, alum-schist, granular limestone and dolomite, sometimes also hornblende-schist, ironstone, and graphite.

13. Mica-schist Formations, with similar subordinate formations to those in the argillaceous mica-schist. In these we include some kinds of gneiss.

14. Gneiss Formations, consisting of gneiss of various kinds in parallel and alternating strata, and containing similar subordinate formations to the micaschists.

15. Chlorite-schist Formations, also containing similar subordinate beds of other rocks.

These formations seem to be the result of a special process of transmutation occasioned by the presence of magnesia.

The fact that in the crystalline schists coal, gypsum and anhydrite are much more rarely met with than in the sedimentary formations, and rock-salt almost never, may, as we have already said, be accounted for by the perishable nature of those rocks. It seems remarkable that conglomerates are also very rarely met with. We should not, however, forget that these only play a subordinate part in the sedimentary formations, where they are usually only of local occurrence. They are, moreover, found in some crystalline schists, as, for instance, in Valorsino and in the Upper Rhine Valley, in the west Alpine district, where they occur in the gneiss and micaschist formations, and pass over by transition into those rocks, their cementing medium having become crystalline, and the pebbles blended with the general mass.

The following is a list of the great geological periods

CHAPTER V.

TRANSITIONS AND TRANSMUTATIONS.

WE have hitherto treated generally of the composition of rocks, their texture, and other outward characteristics, and their formation or origin. It is comparatively easy to describe these phenomena in general terms, but their application to particular rocks in describing and classifying them is a task of great difficulty. One of the principal difficulties of classification is occasioned by the great number of rocks of character varying more or less from the established types. These varieties, in many cases, form series with every shade of divergence from the normal rock until the last member of the series presents a totally different species, coinciding may be with some other normal type. A series of intermediate rocks thus connecting two established types is termed a series of transition; and thus, in the abstract, one type is said to pass into the other; not, however, that any real transition takes place of the actual rock, but merely, as we have said, that two groups are connected together by a chain of rocks partaking partly of the attributes of each.

Transitions of this kind are met with in nature in almost all kinds of rock, in respect alike of their composition, their texture, and their origin. A few instances will suffice for explanation.

1. Transitions in respect of composition are said to take place when in a rock of given character a strange mineral ingredient occurs not usual in rocks of that class, or when an essential ingredient of its composition diminishes or altogether disappears.

For instance, in the case of limestone and dolomite, a rock consisting essentially and principally of calc-spar (carbonate of lime) is a limestone, even though it contain some bitter spar or carbonate of

magnesia; but if enough of the latter enters into. its composition, then the rock will be a dolomite; and an endless variety of rocks are found with very different proportions of those two ingredients, so that it is impossible in many cases confidently to describe them either as limestone or dolomite. These are transition states between those two typical rocks. Again, in the case of gneiss and micaschist, we find first some, and then more felspar entering into the composition of a mica-schist, until at last we obtain a gneiss; or we find less and less felspar in a gneiss, until at last it is reduced to a mica-schist. These and the like transitions may actually be observed in nature side by side, so that in the same mass we may sometimes find at one end a limestone, at the other a dolomite; at one extremity a gneiss, at the other a mica-schist, &c.; but the term transition is employed in this and other treatises in a wider sense to characterise any rocks of intermediate composition, wherever occurring, by means of which a relationship or connection may be traced between any two species of rock. 2. The same kind of transition takes place between rocks in respect of their outward characteristics. The texture of rocks of every kind varies indefinitely from one type to another, without any sharp distinction between the types; thus granite passes over into gneiss in numberless instances where it is more or less foliated in texture; or granite-porphyry passes into porphyritic granite by means of those rocks whose matrix partakes more of the granular than the compact texture; or basalt into dolerite, by those varieties in which the individual minerals are somewhat more separately developed (granular) so as to be partially recognisable.

3. Transitions occur between rocks in respect of their origin or mode of formation. Certain rocks are only the result of a transmutation of others, and the different stages of such transmutation have been distinguished by separate names. Thus argillaceous shale passes over into clay-slate and argillaceous mica-schist; peat into browncoal;