Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/599

 but is the material of the bed polished in some way — and, further, that the detached fruit-like body has many layers of such glazed surfaces, subparallel to each other, and all separated by the ordinary substance of the bed, which thus makes up the body of the so-called fruit. The glazed laminae originate from the somewhat granular nucleus. There frequently occur on the same hand-specimen small patches of similarly glazed surfaces with irregular outlines (Pl. XIX. fig. 12), to which the name Slickenside has been incorrectly applied ; this term is properly employed and should be restricted to designate the smooth and polished appearance produced by enormous friction on the contiguous surfaces of a fault. The structures, however, in shale, to which the name is applied are confined to the stratum in which they occur, and never pass from one surface to the other ; they occupy all positions in the bed, and are isolated or sometimes united at angles more or less acute ; the polished surfaces are frequently crumpled and waved. All these points in their structure show that they are not due to a force external to the bed, and that the polishing has not been produced by the sliding backwards and forwards of the one surface on the other. Dr. Fleming carefully investigated these appearances, and proposed what appears to be a very satisfactory explanation of their origin. He believed them to have been produced in cavities in a comparatively soft plastic matter by the presence of water or gas contained in the cavities, and that the specular aspect was the casting or impression of the fluid substance*.

These limited slickensides, or as I would prefer to call them fluid-casts, occur in rocks which have been at one time or other in a more or less viscous or tenacious condition ; they are found in argillaceous rocks of all ages. So also are the fruit-like bodies which I have described ; I have obtained them from the shale-beds of the Coal- measures in England, Wales, Scotland, and North America, and from the Newer Tertiary clays of Ulm, Wurttemberg ; they have been figured from the Coal-measures of Germany, and from the Permian rocks of Saxony. The granular nucleus which is found in the centre of these fruit-like bodies, was, I believe, the source of the gaseous substance which has left its impress on the glazed surface. This nucleus appears to me to have been a crystalline concretion, which subsequently decomposed ; and the gas then given off spread outwards as it was produced in the planes of stratification, this being the direction of least resistance.

These fluid-casts were figured by Rhode in his ' Beitrage zur Pflanzenkunde der Vorwelt' (1820) pls. vi., vii., ix., and x. The first two plates give a faithful representation of the objects ; but a little play is given to the imagination in plate ix. ; and its unfettered operation is seen in the tenth plate, where the different polished surfaces are converted into the petals of flowers, and the whole are associated with foliage for which a species of Veronica has apparently supplied the design.

The two following species have been based on these fluid-casts : —

vol. iii, p. 170.
 * " Dr. Fleming on the structural characters of Rocks," Proc. Roy. Soc. Edin.