instances prevails to the entire exclusion of any other ingredient, forming a granular quartz rock: it is more abundant in the aggregates of a coarse grain, clay being the chief ingredient in those of a close and fissile texture. They have all an internal stratified structure, which is less apparent in those of a coarse grain, (and in a cabinet specimen scarcely discernible) but which becomes gradually more distinct as the texture becomes finer, and at last the rock graduates into a fine grained slate, divisible into lamina as thin as paper, and having the smooth silky feel and shining surface of the clay slate of a primary country. Alterations of the fined grained slaty varieties with those of the coarsest structure in many successive strata and without any regularity of position, are of constant occurrence, and frequently without any gradation from one structure into the other.[1] In some instances portione of slate are contained in the coarse grained varieties. Scales of mica are frequent, and they all contain oxide of iron in greater or less proportion, and to the different states of this oxide their various colours are, no doubt, to be ascribed. The prevailing colours are reddish brown and greenish grey, and there are many intermediate shades and mixtures of these colours. Some of the slaty varieties are of a purplish hue, and this is occasionally spotted with green. Of the specimens I collected, those of a coarse grain and of a dark reddish brown colour, do not effervesce with acids; those of a pale reddish brown colour, and of a greenish grey colour, all effervesce, and some of them briskly. None of the varieties of slate shew any signs of effervescence. The magnet was not affected by any one of the series. I did not discover a trace of any organic body in either
- ↑ Similar alternations of clay slate with grauwacke have been observed at Crinan and at Aberfoyle, by Dr. Mac Cullech. (Transaction of the Geological Society, Vol. II. p. 441, 447) and by Mr. Jamseon in the Pentland-hills. (Mémoire of the Wernerian Natural History Society, Vol. II. Part. I. p. 184.)
