Seamed Structure.—T hose rocks in which parallel scams occur, that do not extend throughout the whole bed, are said to have the seamed structure. The follow ing are the different kinds of this structure : I. Polyedral. 2. Lamellar. 3. Globular.
Polyedral.—In this kind of structure, the rock is divided into columns. The columns are sometimes re gular, sometimes approach to the globular form, and occur even curvated. They are from a few inches to many fathoms in length. In the islands of Staffa and Eigg, and on the Craig of Ailsa, there are admirable examples of this kind of structure. These columns are sometimes collected into groups, and such groups are often separated from each other by seams, or rather rents, which render them more distinct. Such groups may be considered as immense distinct concretions. The columns of such a group often tend towards a centre ; others are parallel or perpendicular ; some are horizontal ; and all this variety sometimes occurs in the same hill. The columns are sometimes jointed, so that the convex extremity of the one column is fitted to the concave extremity of the other ; and these columns are usually composed of globular distinct concretions. These globular concretions are composed of curved lamellar concretions. The spaces between the different globular concretions are composed of a looser matter than the concretions themselves ; and it is by the falling out of this less compact substance that the structure of such columns is first developed.
No rock shows this kind of structure more distinctly than basalt : in it we have all the varieties of the seamed structure, from the smallest, which is the lamellar dis tinct concretion, to the largest, which is formed by the grouping of columns.
This kind of structure occurs also in porphyry, green stone, and lava.
2. Tabular.—ln this kind of structure, the rock is divided into tables a few inches thick, and of small ex tent. These tables frequently change their direction, and then the bed of rock is divided into many groups or systems of tables, having in each group a different direction.
Their inconsiderable length, frequent change of direc tion, and the even surface of the planes, distinguish them from strata. Basalt, in the lower parts of an individual deposition, has often this kind of structure. At first sight it is not unlike stratification. It also occurs in columnar porphyry.
3. Globular.—In this kind of structure the rock is either wholly or partially disposed, or arranged into globes or balls of various magnitudes. These balls arc either simply granular, as in granite, syenite, green stone, or sand-stone ; granular, and lamellar, as in the syenite of Corsica ; compact, as in basalt ; porphyritic, as in porphyry ; or radiated, as in limestone. Globular formations are sometimes net with in rocks and mine rals having a less crystalline aspect. As examples, we may mention balls of hornstone in limestone, and masses of flint in chalk.
The roundish balls of granite found dispersed over low countries have been considered as boulder or rolled stones, and many theories formed to account for their transportation. The granite of the island of Arran pre sents this kind of structure.
Structure of Formations.
By a Rock formation, we understand a determinate assemblage of similar or dissimilar beds or rock masses, which are characterised by external and internal rela tions as an independent whole; that is, as an unity in the series of rock formations. These masses are either simple or compound. When the mass is uniform through out, as is the case with limestone or sandstone, it con stitutes what may be denominated a simple formation. Granite, gneiss, and mica-slate, are also of this kind. When dissimilar masses occur in a formation, as in the case of coal and secondary trap, it is said to be a compound formation.
Similar rocks are often repeated in very different periods. Each of these individual depositions is a par ticular formation, and the whole is denominated a series or suit offormations. Thus there is a limestone, a por phyry, a granite suit, Ste.
It is, Werner remarks, a determinate character of certain formations, to constitute the principal mass of the mountain in which they occur: this is the case with gneiss, clay-slate, porphyry, and others. With other formations, on the contrary, it is as essential a character to occur only in single beds in the others, and these are said to be imbedded. Primitive porphyry, limestone, and many others, aro of this kind. When such individual beds occur in different principal formations, (that is, arc not confined to a single one,) as primitive limestone and primitive trap in gneiss, mica-slate, and clay-slate, and always continue the same, notwithstanding the differ ence of the rocks in which they are embedded, they are to be considered as independent formations. If, on the contrary, they are confined to one formation, and are connected with no series or suite of formations, they arc said to be subordinate to it. Alum, slate and Lydian stone n clay-slate, are examples of this kind. Beds comp sed of various minerals sometimes occur in differ ent rocks. These minerals are usually those of which the rock is principally composed, irregularly mixed, or are simple minerals. Such beds arc usually very irre gular, do not extend through the whole rock, and in general exhibit apparently great irregularity in all their relations. They have been, but rather improper ly, de nominated foreign beds. \Vhen single beds arc well distinguished by the kind of stone of which they are com posed, and if their composition shows certain peculiari ties, (as, for example, the determinate presence of me tallic fossils, &c.) they are referred to the particular re positories. Beds of iron-pyrites and magnetic ironstone are of this kind.