VIII.—On a recent Formation of Freshwater Limestone in Forfarshire, and on some recent Deposits of Freshwater Marl; with a Comparison of recent with ancient Freshwater Formations; and an Appendix on the Gyrogonite or Seed-vessel of the Chara.

Abstract

The freshwater limestone which it is now my chief object to describe, differs considerably in the circumstances under which it occurs, from any similar production that has yet been noticed. In order that these circumstances may be fully understood, I shall give a brief geological sketch of that part of the county of Forfar in which the limestone has been discovered. The county may be regarded as divided into three principal districts; that of the Grampians, that of Strathmore, and that of the Sidlaw Hills. The granite of the Grampians skirts the northern frontiers of the county, and is followed to the south by gneiss, mica-slate, clay-slate, and greywacké, each of these formations in succession occupying a tract of considerable extent, and each observing a north-east and south-west direction. Immediately on the clay-slate rests a series of strata consisting of red and green shale, of gray micaceous and slightly calcareous sandstone, and of red schistose sandstone, this series being subject in all its parts to be interfered with by rocks of the Trap family. This, which I shall call the series of inferior sandstone, has not yet been ascertained to contain organic remains; but in other leading respects it resembles that assemblage of strata which in England, and particularly in the counties of Hereford, Gloucester, and Monmouth, is so intimately connected with transition limestone. The inferior sandstone dips beneath the old red-sandstone-conglomerate which flanks the Grampians, and emerges again on the south side of the valley of Strathmore, there forming