We have thus a geognostic series of structure from hand specimens, which can be examined in the closet, to that which can only be known by travelling through many countries.
We shall now describe the different kinds of struc ture, in the order already mentioned.
Structure of Mountain Rocks Mountain Rocks or Stones, are those mineral masses of which the greater portion of the crust of the earth is composed. Minerals, or mineral aggregates, to have the true characters of mountain rocks, must oc cur not only in great masses, but frequently, and pre sent in their structure and composition such characters as will serve to distinguish them, and make them known in whatever situation they may be found.
They are either simple, or aggregations of simple minerals. In the one case, they are denominated Sim ple mountain rocks ; in the other, Compound mountain rocks.
Simple mountain rocks are either compact, slaty, or granular. Clay-slate, limestone, and serpentine are ex amples of simple mountain rucks.
When minerals occur disseminated through simple mountain rocks, they are to be considered as accidental, and do not entitle us to consider such varieties as dis tinct species of rock. Thus garnets, imbedded in gra nite or mica-slate, are accidental mixed parts.
Compound mountain rocks are either conglomerated or aggregated. This distinction is founded on the mode of their formation. The grains or masses in the conglo merated mass, according to some naturalists, have not been formed on the spot where they are now found, but have been carried thither, and connected together by a cement. To this class belongs sandstone, pudding stone, and different kinds of fragmented or brecciated stones.* In aggregated mountain rocks or stones, their present structure is their original one, and the parts of which they are composed have been formed on the spot where we now find them. Their parts are immediately con nected together ; hence this structure is termed aggre gated.
Aggregated mountain rocks are either dcterminately or indeterminately aggregated. The parts in the inde terminately aggregated, are irregularly and confusedly joined together. This kind of structure occurs in those
varieties of serpentine, where limestone and serpentine are so conjoined that it is difficult to say which predo minates, and where the one sometimes encloses the other. The Campan marble, which is an irregular mixture of limestone and steatite, and many Cipolin marbles, which contain veins and patches of green talc, are of the same description.
The determinately aggregated structure, presents a number of subordinate differences. It is either simple or double aggregated. The simple aggregated contains two subordinate kinds. In the first, the minerals are connected together in such a manner that one serves as a basis for the other, which is included in it ; and it also contains two subordinate kinds. These are deno minated the porphyritic and amygdaloidal. In the se cond, all the parts are immediately connected, or joined together ; and here we have also two subordinate kinds, the granular and slaty.
The double aggregated includes five subordinate kinds: These are, I. Granular slaty. 2. Slaty granular. 3. Granular porphyritic. 4. Slaty porphyritic ; and, 5, Porphyritic and amygdaloidal. The first four kinds of double aggregated structure comprehend one structure in another, so that, as the denominations intimate, a smaller structure is contained in a greater. In the fifth, or last kind, one does not include the other ; but, as the denomination expresses it, they are placed near or beside each other.
We shall now describe each kind in particular, Simple Aggregated Structure.
1. Porphyritic structure.—When one of the constitu ent parts of the mountain rock is disseminated through a basis, in the form of grains or crystals, the rock pre senting this appearance is said to be porphyritic. Com mon porphyry, porphyry slate, and gypsum contain ing crystals of quartz, may be mentioned as examples of this kind of structure. The crystals or grains are here of cotemporaneous formation with the basis, and not mechanically mixed, as some have maintained.