2. Atnygdaloidal structure.—When vesicular cavities are dispersed through a basis or ground, and appear empty, encrusted, half filled, or completely filled, such a structure is denominated amygdaloidal.
The rock named aulygdaloid, is a principal example of this kind of structure. Its basis approaches more or less to basalt or greenstone ; when it is much iron shot, it becomes harder and more solid. It is alleged, that while the amygdaloidal rock was still soft, bubbles of air were disengaged, which being prevented escaping by the viscosity of the mass, various shaped cavities, often however of an amygdaloidal shape, were formed. Water holding in solution the various minerals met with in these amygdaloids, is alleged to have traversed the rock, penetrated into the empty vesicular cavities, and to have deposited on their walls its mineral contents. Hence it is maintained, that the amygdaloidal portions are of posterior origin to the basis in which they are con tained. It cannot be questioned, that some amygdaloidal structures have originated in this manner, but many others arc certainly of cotemporaneous foimation with the rock in which they are contained. The minerals that usually occur in these vesicles, are lithomarge, zeolite, steatite, chalcedony, agate, heavy spar, and calc-spar. Those filled with agate and chalcedony, present many interesting phenomena.
3. Granular structure. In aggregated rocks, some kinds are formed by the immediate aggregation of different minerals, which are intimately joined together, either by the power of cohesion, or by mutual penetration or in terlacement. These minerals are generally in grains, and may be regarded as imperfect crystals. Granite affords an example of this kind of structure.
4. Slaty atructure.—Thc slaty structure in rocks emu posed of nifferent minerals differs from the granular in this circumstance, that the constituent parts are flat, having considerable length and breadth. but inconsi derable thickness. Mica slate, which consists of small plates of quartz and small plates of mica placed upon each other, is an example of this kind of structure.
Double ?lggregated Structure.
Under this head we include those varieties where two structures occur together. The following are the
different kinds.
I. Granular-slaty structure.—Gneiss affords a good ex ample of this kind of structure ; the granite and felspar are in grains, and immediately aggregated together, which forms the granular structure ; and these aggregations are generallydisposed in plates, which arc interposed between the plates of mica, and hence the slaty structure.
2. Slaty-granular structure—ls slaty in the small, and granular in the great. The only example of this kind of structure is the topaz-rock of Schneckenstein, in Voightland, in the electorate of Saxony. It is com posed of large granular masses, which have a slaty structure ; the slates consist of fine granular quartz, thin columnar schorl, and small granular topaz. In the drusy cavities that occur between the large granular concre tions,lithornarge, crystals of topaz, and schorl, sometimes occur ; the schorl, however, is but rarely crystallized.
3. Granular-porphyritic structure.—This kind of structure is granular in the small, and porphyritic in the great. When large and distinct crystals of felspar appear dispersed through a granular base, as is often the case with granite, sienite, and greenstone, this kind of structure is formed.
4. Slaty kind of structure is slaty in the small, and porphyritic in the great. The basis is slaty, and the porphyritie structure is formed by in terspersed crystals or grains of minerals different from the basis. Mica-slate, when it contains grains or crys tals of garnet is said to have a slaty-pot phyritic structure.
5. Porphyritic and ?magdaloidal.—Here two kinds of structure are placed together, not included in each other, as in the preceding kinds. It occurs in many amygdaloidal and porphyritic stones. When amygda loid contains, besides the elliptical-shaped masses, also crystals of hornblende and mica, it is then said to have an amygdaloidal and porphyritic structure ; the amyg daloidal being the predominant. In Basalt, on the con trary, where the two kinds of structure sometimes occur, the porphyritic is the predominating. Green Porphyry, although rarely, sometimes possesses this kind of double structure.