Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/198

 It is usually much weathered, its decomposition giving rise to a red sandy soil, whence the district derives its name.

At Carnecome is a large standing block of clay porphyry, of an ash-grey colour, containing the same ingredients as that above described.

Pitchstone porphyry and pearlstone porphyry occur apparently as subordinate members in this formation.

Two large masses of each variety may be seen at the bridge across the Loonburn, on the road from Connor to Doagh.

Pitchstone Porphyry of Sandy-brae.

The most sound and interior part is bluish black, possessing a splendent and vitreous lustre: its specific gravity is 2,52; a little nearer to the surface it passes to olive green, with a waxy texture, and has a specific gravity of 2,50; at the surface it appears yellowish green and rather earthy, with a specific gravity of 2,40; all these different shades sometimes occur in the same specimen, and appear evidently to result from decay.

Engaged in the basis round concretions of smoky and vitreous quartz, with lamellar crystals of glassy felspar possessing a yellowish tinge occur. Common opal, nearly approaching to precious opal, accompanies this pitchstone porphyry; it is either disseminated in plates or in small strings.

Pearlrtone Porphyry of Sandy-brae.

The texture is formed of vesicular and distinct concretions loosely coherent, of an elongated or irregular shape, the angles being smooth