- Class 2. Earths.
- Ord. 3. Aggregated.
- Gen. 6. Silex.
- Spec. 2. Grains of Silex more or less agglutinated.
- Div. 3. Amorphous
- Spec. Char. Fracture granular.
- Siliceous sandstones. Kirw. v. 1. 364.
- Cos fraibilis. Linn. Syst. v. 3. p. 63. 9.
- — coagmentata. Linn. Syst. v. 3. p. 63. 10.
- Quartz aranacée agglutinée, ou Grés. Haüy, v. 4. 464.
Sandstones may be said to be composed chiefly of quartz in smaller or larger particles, which, according to Kirwan, should not exceed one third of an inch in diameter. In the representation of such stones are not primitive, it is though of much utility to put those which have impressions on their surfaces of plants, shells, or other things formerly organized, that while we acquire a common idea of the substance, it may help geological purposes, which will be found extremely essential in mineralogy, as it leads to the æra of formation of different strata, distinguishing by such helps the more recent from the most remote.
The upper figure is chiefly composed of irregular whitish grains of quartz, cemented to each other by a sort of agglutination of its own particles, and in some parts with oxide of iron, which gives it the brownish tinge: it has a few specks of mica, and a very little decomposed feltspar. This was sent me by the Rev. Mr. Harriman from Durham.
The lower figure is perhaps the coarsest sort of sandstone, of much the same ingredients, but of a looser texture, with more decomposed feltspar, and was given me by Lady Wilson, who brought it from Walmington in Cumberland. The coasreness of the stone shows plainly that it could not have been formed by human contrivance with the present beautiful ornament, but that it is a natural production, which equals in simplicity and elegance some of the most admired ornaments of antiquity, and may, like them, give an useful hint to modern architects.
The impressions seem to be like the leafy scales of the stem of some plant yet unknown to us. They are most like some foreign Euphorbia or Cactus.