Structure of Mountain-masses and of Strata.
We understand by the structure of mountain masses, those regular divisions which many of them exhibit through a considerable extent of rock. There are two kinds of this structure ; the stratified structure, or that of strata ; and the seamed structure, or that of distinct concretions.
Stratified atructure.—When a mountain, mountain mass, or bed composed of one species of rock, is divided by means of parallel scams into mass, s, whose length and breadth are greater than their thickness, or into what may be denominated Tabular Masses, which extend generally through the whole mountain, it is said to be stratified, and the individual masses are termed strata. Of this kind of structure we have instances in Granite. Limestone, Clay•slate, and Mica-slate. The seams that separate the strata are named strata-seams, or seams of stratification. On a general view, we say the globe is composed of formations ; formations of beds ; beds of strata ; and such strata as are slaty, of layers, or slates.
Strata vary very much in their position. The exami nation of this appearance of strata is of great importance to the geognost and mineralogical geographer.
In determining the stratification of a rock, we have to observe, 1. The direction of the strata. 2. Their inclina tion. 3. Their thickness. 4. The differences in their direction and inclination.
The direction is the angle which the stratum makes with the meridian, and is determined by the compass ; or viewing the strata as planes, the direction is that of a horizontal line, drawn on the surface of the plane ; therefore, to determine the direction, is to indicate the point of the compass towards which this line is directed. It is always at right angles to the dip.
The inclination is the angle which the stratum forms with the horizon, and is determined by the quadrant.
The dip is the point of the compass towards which the stratum inclines. If we know the dip, the direction is given, because it is always at right angles to it. Thus, if a stratum dip to the east, its direction must be north and south ; if it dip to the north, its direction must be east and west. But we cannot infer the dip from the
direction; because a stratum, whose direction continues the same, may dip in opposite directions ; thus, a stratum ranging from north to south may dip either to the east or to the west.
Strata are frequently variously waved, thus producing changes in their direction ; but these changes in direc tion are, in general, subordinate to the general direction. It is therefore of importance, in making observations with regard to the position of strata, to distinguish the gene ral direction and inclination, from the partial. To effect this, we must take the results of a number of particular observations, and compare them together ; and those similar angles, which are the most numerous, are to be considered as expressive of the general inclination and direction. It sometimes happens, that this general posi tion has also its variations ; these must also be attended to and noted. An acquaintance with the shape of a moun tain group will assist us very much in such investiga tions, as it is intimately connected with the general dis position of the stratification of the masses of which it is composed. It is also of importance to know the fall or declivity of a mountain group, as its direction and incli nation are frequently conformable, particularly in the older formations, with that of the superimposed masses.
It is often very difficult to determine, whether the rock we are examining be stratified or not, and when the seams of stratification are hid, to know the direction of the strata. The following observations will be useful in removing some of the difficulties attending such investi tions.
1. Strata are almost always parallel with the slaty structure of the stone. In certain porphyritic granites, the crystals of felspar appear to lie parallel with the strata ; the latter character, however, is by no means so decisive as the former.