Structure of the Crust of the Globe.
The fourth and last kind of structure we have to describe is by far the most extensive and important. It is the structure of the crust of the globe itself, in so far as it is composed of rock-formations of different magnitudes, laid over each other in certain directions.
Under the five following heads, Werner comprehends every relation respecting the extent and relative posi tion of formations in general.
1. The original extent of formations.
2. Their present extent and continuity.
3. The position and direction of the strata of forma tions, in respect to the fundamental rock.
4. The direction of the strata themselves, without re ference to the fundamental rock.
5. The relation of the outgoings of the strata to the exterior of the mountain.
It may be previously remarked, that when one forma tion lies on another, it is said to rest on it, and the rock on which it rests is termed the fundamental rock ; and the plane which separates the fundamental rock from the formation that covers it, is denominated the plane of separation, which is always parallel with the seams of the strata. See Plate CCCXCV. Fig. 1.
1. The Original Extent of Formations.
Werner observes, that the greater number of forma tions appear to have been very widely deposited ; and these he denominates Universal Formations. Some others he considers as haviug been partially deposited, and these he terms partial or anomalous formations.
Universal Formations extend around the whole globe, (not, however, without interruption.) and con stitute by far the greater part of the mass of which its crust is composed. Almost all the primitive transi tion, and secondary formations, are universal deposi tions ; of these, we may mention granite, gneiss, por phyry, and limestone.
Partial formations occur only in particular places, and appear to owe their existence to particular and lo cal causes, for example, to depositions in lakes. La manon is of opinion, that the gypsum formation around Paris was formed from the waters of a lake which for merly occupied that part of France ; and the exten sive local formation in the Isle of Wight, and the opposite coast of England. appears also to be a lake formation.
At Wehraw, in Lusatia, there is an example of a partial formation. It consists of sandstone, limestone, bitumi nous-shale, and iron-clay ; and these rest on loose sand. The sandstone resembles, in many respects, that found in other sandstone formations ; yet it does not belong to any of them, as is evident from its position, and the rocks with which it is associated. Werner conjectures that it may have been formed by a small and partial flood.
The examination of these partial appearances is of much importance, not only in extending our knowledge of the variety of formations, but in connecting the his tory of the earth more nearly with that of man.
2. The Present Extent and Continuity of Formations The present extent and continuity of formations is very different from what it was originally. We find them either extended uninterruptedly over great tracks of country, or they appear isolated, of little extent, and frequently resembling partial formations. In the one case, they are said to he unbroken, in the other broken. The broken formations occur in small detach ed masses ; and these have peculiar denominations, ac cording to their position and shape.
When detached portions occur on the summits of hills, these are called caps, Fig. 2. a. When portions occur, filling up holloe spaces between mountains, they arc denominated upfillings, Fig. 2. b. And when a portion occurs only on one side of a mountain, it is said to be shield formed, Fig. 2. c. or to have the shape of a shield.
3. The Position and Direction of Strata in regard to the Fundamental Rock.
When strata have the same direction as the funda mental rock, they are said to be conformable with it, Fig. 3. a. if the direction is different, they are said to be unconformable. If they differ only in direction, we say that they are simply unconformable, Fig. 3. b.; but if they differ not only in direction, but lie over the ends of the strata of the fundamental rock, they are said to be unconformable and overlying, Fig. 3. c. Overly ing strata occur Inure frequently than simply uncon formable, and have far greater extent.