One substance explains another. The present Micaceous Sandstone is by favour of R. Ferguson, esq. (gathered in Dumfriesshire). The peculiarity and beauty of it are indisputable, and I think it highly instructive in a geological point of view; the forming of the circles depending upon circumstances that may help to account for those in the Eyed Agate, (tab. 160,) and for other similar constructions. It may appear that substances that give changes of colour by different degrees of oxidizement, are more or less affected by the quantity or spreading of the moisture producing these circles, such as, in more familiar cases, coloured or dirty water on a plaster wall, &c.
The Sand is occasionally mingled with small particles of Mica that give somewhat the appearance of Avanturine or Lepidolite. However, the shining particle-like appearance is in the latter said by some to be occasioned by two facets meeting, and allowing of minute reflections of the light (But of this hereafter in Exotic Mineralogy.) The circles, &c. in the Corsican Orbicular Sienite, figured in that work, tab. 2, probably depend upon this formation in a coarser and on a larger scale, which would best agree with the substances that compose it.