4. Direction of the Strata themselves, without reference to the Fundamental Rock.
Strata are either straight, that is, disposed in one direction on the fundamental rock, Fig. 4. or they turn around it, and inclose it ; in this latter case they are said to be mantle formed, or mantle-shaped, Fig. 5. b. If the strata are not only wrapped around the fundamental rock, but also cover its extremities, they are said to be saddle shaped, Fig. 6.
When the upper part of the saddle-shape is carried away, the mantle•shape is formed.
Strata arc sometimes concave, and they are then said to be basin-shaped, Fig. 7. a. ; Fig. 10. but if the conca vity be oblong, they are named trough shaped, Fig. 8 6. In the concave, that is, the basin and trough-shaped, and in the convex, that is, the saddle and mantle-shap ed strata, the outgoings' form circles : in the concave, the outer and greatest circle, according to \Verner, is the oldest ; in the convex, on the contrary, the outer and largest circle is considered to be the newest.
S. The Relation of the Outgoings of the Strata to the terior of the Mountain.
We have to consider first, the relation of the outgo ings of strata to mountain-masses of considerable ex tent ; and, secondly, to mountain caps.
In mountain-masses, the strata either cover each other completely, or the outgoings are open and ex posed. When the outgoings are exposed, the newer strata have a rising or sinking level, Fig. 9.
There are three different kinds of mountain-caps : In the first, the cap rests on a fundamental rock, the seams of the strata are parallel with the plane on which the strata rest, and these are unconformable and overlying, Fig. 10. a. The second kind of cap is form ed by a rock rising through the surrounding strata, Fig. 1 I. b.: and the third kind of cap is formed by por tions of harder beds remaining after the superincum bent and adjacent and softer strata have been carried away, Fig. 12. c.
In the first, the rock is unconformable and overly ing. The secondary-trap and porphyry formations af ford numerous examples of this kind of mountain-cap.
In the second, the newer strata are mantle-shaped. Granite often occurs in caps of this kind.
In the third, all the strata are conformable, so that the subjacent and superincumbent strata have the same di rection with the bed which forms the cap. Primitive greenstone in clay-slate, sometimes forms caps of this kind.