26. _Schists_.--Under this term comes a great variety of crystalline rocks which all agree in having a foliated texture--that is to say, the constituent minerals are arranged in layers which usually, but not invariably, coincide with the original bedding. Amongst the schists come _mica-schist_ (quartz and mica in alternate layers); _chlorite-schist_ (chlorite with a little quartz, and sometimes with felspar or mica); _talc-schist_ (talc with quartz or felspar); _hornblende schist_ (hornblende with a variable quantity of felspar, and sometimes a little quartz); _gneiss_ (quartz, felspar, and mica). 27. _General Character of Metamorphic Rocks._--All these rocks betray their aqueous origin by the presence of more or less distinct lines of bedding. They consist of various kinds of arenaceous and argillaceous deposits, which, under the influence of certain metamorphic actions, to be described in the sequel, have lost their original granular texture, and become more or less distinctly crystallised. And not only so, but their chemical ingredients have in many cases entered into new relations, so as to give rise to minerals which existed either sparingly or not at all in the original rocks. Frequently, it is quite impossible to say what was the original condition of some metamorphic rocks; often, however, this is sufficiently obvious. Thus, highly micaceous sandstones, as they are traced into a metamorphic region, are seen to pass gradually into mica-schist. When the bedding of gneiss becomes entirely obliterated, it is often difficult to distinguish that rock from granite, and in many cases it appears to pass into a true granite. 28. _Granite_ is a crystalline compound of quartz, felspar (usually potash-felspar), and mica. Some geologists consider it to be invariably an igneous rock; but, as just stated, it sometimes passes into gneiss in such a way as to lead us to infer its metamorphic origin. There are certain areas of sandstone in the south of Scotland which are partially metamorphosed, and in these we may trace a gradual passage from highly baked felspathic sandstones with a sub-crystalline texture into a more crystalline rock which in places graduates into true granite. Granite, however, also occurs as an igneous rock. 29. _Syenite_ is a crystalline compound of a potash-felspar and hornblende, and quartz is frequently present. _Diorite_ is a crystalline aggregate of a soda-felspar and hornblende. Both syenite and diorite also occur as igneous rocks. There are a number of other metamorphic rocks, but those mentioned are the most commonly occurring species. IGNEOUS ROCKS.