seem to run out from this at various angles. Faults are of all geological ages. Some date back to a most remote antiquity, others are of quite recent origin; and no doubt faults are occurring even now. In the following diagram, the strata, _a, a_, have been faulted and planed away before the strata, _b_, were deposited. Hence, in this case, it is evident that if we know the geological age of the beds, _a_ and _b_, we can have an approximation to the age of the fault. If the beds, _a_, be Carboniferous, and those at _b_ Permian, then we should say the fault was _post-Carboniferous_ or _pre-Permian_. [D] The degree of inclination is very variable. It may occur at almost any angle up to vertical. But, as a rule, the hade of the more powerful faults is steeper than that of minor displacements. [Illustration: Fig. 18.--Ground-plan of Large Main Fault and Minor Displacement Fissures.] [Illustration: Fig. 19.--Faulted Strata covered by undisturbed Strata.] 51. _Metamorphic and Igneous Rocks--mode of their occurrence._--In the foregoing remarks on the structure and arrangement of rocks we have had reference chiefly to the aqueous strata--that is to say, the _mechanically_, _chemically_, and _organically_ formed rocks. We were necessarily compelled, however, to make some reference to, and to give some description of, certain structures and arrangements which are not peculiar to aqueous strata, but characterise many metamorphic and igneous rocks as well. To avoid repetition it was also necessary, while treating of _joints_, &c., to give some account of certain structures which are the result of metamorphic action. But, for sake of clearness, we have reserved special account of the structure and mode of occurrence of metamorphic and igneous rocks to this place. After what has been said as to the structure and arrangement of aqueous strata, it is hardly needful to say much about the crystalline schists. These the student will understand to be merely highly altered aqueous rocks,[E] in which the marks of their origin are still more or less distinctly traceable. As a rule, metamorphic strata are contorted, twisted, and crumpled, although here and there comparatively horizontal stretches of altered rocks may be observed. The regions in which they occur are often hilly and mountainous, but this is by no means invariably the case. The greater part of the mountainous regions of the British Islands is occupied by rocks which are more or less altered; the more crystalline rocks, such as mica-schist, gneiss, &c., being abundantly developed in the Scottish Highlands, and in the north and west of Ireland; while those which are less altered cover large areas in the south of Scotland, and in Wales and the north-west of England. Throughout these wide areas the rocks generally dip at high angles, and contortion and crumpling are of common occurrence. The finer-grained clay-rocks also exhibit fine