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66 INTERNAL STRUCTURE. the outer and greateft circle is the oldeft ; in the convex, on the contrary, the outer and largeft circle is the neweft. 5. The relation of the outgoings of the Jlrata to the exterior of the mountain. We have to confider firft, the relation of the outgoings of ftrata to mountain-malTes of con- liderable extent ; and, fecondly, to mountain- caps. In mountain-malTes, the ftrata either cover each other completely, or the outgoings are open and expofed. When the outgoings are expofed, the newer ftrata have a rifng or finking level. There are three different kinds of mountain- caps: In thefirfi, the cap refts on a fundamental rock: the feams of the ftrata are parallel with the plane on which the ftrata reft, and thefe are un- conformable and overlying : the fecond kind of cap is formed by a rock riling through the fur- rounding ftrata: and the third kind of cap is formed by portions of harder beds remaining after the fuperincumbent and fofter ftrata have been carried away. In thtfirfi, the rock is unconformable and over- Iying. The Flcetz-trap and Porphyry formations afford numerous examples of this kind of moun- tain-cap. In the fecond, the newer ftrata are mantle-ftiaped. Granite often occurs in caps of this kind. In
