LECTURE IV.

Minerals entering into the composition of RocksDifferent forms of Rocks-Masses, Beds, Strata, Nodules, Veins-Internal Structure of Rocks-Laminar, Fibrous, Spheroidal, Prismatic, Veined, Cavernous, Amygdaloidal, Aggregate, Granular, Porphyritic-Texture--Fracture--Hardness-Color-Frangibility

-Lustre-Transparency-Specific Gravity-Action of

Acids.

Having cursorily glanced at the present appearance of our globe, and at the changes now going forward, from various causes, on its surface, we proceed to examine the rocks that enter into its composition. Before entering upon a description of them, it is necessary, however, to premise that a knowledge of Mineralogy is in some degree essential to the study of Geology, of which it has been, not unaptly, styled the Alphabet. Nearly all mineral substances are found either as constituents of rocks, or as occasional substances imbedded in them.

There are not many minerals entering into the composition of rocks: The following enumeration

is perhaps complete. For a knowledge of these substances I must refer you to some treatise on mineralogy-and none is more deserving of commendation than that of Professor Cleveland, of

A larger number of substances occur imbedded in rocks, in such quantities as materially to alter the character of the matrices, viz.

It is not too much to say that every variety of

mineral is found imbedded in, or connected with some rock.

Rocks in different positions assume different forms; which may be reduced to five, viz.

IRREGULAR MASSES,

BEDS,
STRATA,

NODULES,
VEINS,

Irregular Masses, may be of any size; and often constitute mountains, as is the case with granite, serpentine, porphyry, and the overlying rocks, as trap, &c.

Beds are of various sizes; often running into irregular masses: They are straight, or curved, and frequently intersected by joints, so as to assume a cuboidal appearance. Few rocks assume this form: those most disposed to it are granite, porphyry, syenite, and greenstone or hornblende. This distinction is considered, by Dr. Macculloch, as being practically the most easy, and perhaps the only one necessary for the student. Beds and irregular masses often give out veins that penetrate the adjoining rocks.

Strata have been confounded with beds, but they are generally much larger, and usually are more extensive in two dimensions than in the third: so that strata may be considered as immense beds with the upper and lower surfaces parallel, in most

cases; but occasionally meeting at a very acute angle. Strata do not necessarily preserve the same thickness, and often vary in the course of a few yards; nor are they always straight; frequently being contorted and flexed into larger or smaller curvatures, which may be either parallel or transverse to the plane of stratafication.

This form of rocks is of various extent, sometimes being discernible only for a few hundred yards, and at others being well defined for hundreds of miles. Strata are found at all angles with the horizon, and in all relative positions to each other and the adjoining rocks.

A rock is not necessarily of the same modification through the whole extent of a stratum, as the texture may vary to the widest limits of fine and coarse.

Strata never send off veins into adjoining rocks.

Nodules, or imbedded irregular masses, is a term lately adopted to include rocks which are not stratafied nor disposed in pseudo strata (beds), and which do not resemble in their connections other large irregular masses. The forms of the Nodules are various; and they are usually imbedded in the stratafied rocks; but occasionally in granite. The size varies from a foot to a mile. Limestone.

Serpentine and compact Felspar alone have been found of this rare division of form.

Veins are known by their filamentary forms, and by intersecting all other forms of rocks and each other. As this is an important subject and one that has given rise to much Geological disputation, it may be well to enlarge upon it; premising that the school of which Hutton was the founder, consider veins to have been filled from beneath by the action of fire while the disciples of Werner maintain that they were filled from above by aqueous solution and infiltration: all acknowledging that they occupy places or fissures originally open.

Veins are simple, or they exhibit branches or ramifications: the latter are more generally met with in Granite.

The size of veins varies from a mere thread to several hundred feet in breadth, being smallest usually in the primary rocks, and from one foot to several miles in length.

The relative position of veins is extremely diversified, intersecting rocks in all directions, forming every possible angle with the horizon, and dipping to every point of the compass the course of a vein being straight or flexed.