THE STRUCTURE OF SANDSTONE.[1]
AS AFFECTING ARCHITECTURAL AND ENGINEERING WORKS.
The native stones we Liverpool architects have at command are all sandstones belonging to the geological division called the Trias, or, in older phraseology, the “New Red Sandstone,” which lies above the coal-measures. The term “New Red” was given to distinguish these rocks from the “Old Red,” which lies below the Mountain Limestone, the lowest division of the carboniferous rocks. It is, perhaps, needless to remark that the “New Red” is not always red; sometimes it is yellow, at others, like some of the Storeton stone, white. These red rocks occupy a large part of Lancashire and Cheshire, and especially in the latter county give the characteristic scenery which distinguishes it. The escarpment of the Peckforton Hills of which Beeston Castle Hill is an outlier, and that at Malpas, farther south, gives rise to some very beautiful scenery; and again at Grinshill and Hawkstone, in Shropshire, we have a repetition of much the same kind of landscape. It will be necessary for my purpose to say briefly that these red rocks have been divided into the “Bunter” and “Keuper”; the lower division, the Bunter, occupying most of the ground about Liverpool; the upper, the Keuper, being more developed on the Cheshire side. All these sandstones are not fit for building purposes, and those that are so used differ considerably in their durability. It is my object in this short Paper to show upon what the perfection or imperfection of the various stones for building purposes depends—a matter of great moment to an architect or engineer who is desirous that his work should last.
Sandstones, or, in masons’ language, “free-stones,” from the freedom with which most of them are worked when freshly taken from the quarry, are plastic or sedimentary rocks. That is, they are composed of separate particles which have once existed as sand, like that we see on our own shores, or in the sand dunes of Hoylake or Crosby. Sandstones are usually more or less laminated, and are stronger to transverse stress at right angles to their natural bedding than in any other direction, a fact recognized in every architect’s specification, which states “all stones must be laid on their natural bed,” a direction that unfortunately sometimes begins and ends in the specification. The cause of the superior strength is not, however, generally understood.
I have devoted some considerable time to an investigation of the internal structure of sandstones, which I have communicated from time to time to various scientific societies and publications, and will now briefly explain it in a manner I judge will be most likely to interest architects and engineers. The particles or grains of which the rock is built up are of various forms and sizes, from a thoroughly rounded grain, almost like small shot, to a broken and jagged structure, and to others possessing crystalline faces. These grains, most of them possessing a longer axis, have been rolled backwards and forwards by the tides or by river-currents. The larger grains naturally lie on their sides when freshly deposited, with their axes in the plane of bedding; the smaller and more rounded particles naturally tend to occupy the interstices between the others, and in this way rude divisional planes or laminations are formed. Each layer forms a sort of course like coursed-rubble in a wall, and by the necessities of deposition a certain rude geometric arrangement results, by which the particles of the future rock overlap each other, and thereby gain what is known to architects as bond.
But, so far, this is only like “dry walling,” the mass wants cementing together to make it solid. The cementing process happens in this way in our rocks, which are almost purely silicious: Water containing a minute quantity of carbonic acid in solution, which most rain-water does, especially when it comes into contact with decaying vegetation, has the power of dissolving silica to a slight extent. This is proved in various ways, and is shown in the fact that all river water contains more or less silica in solution.
The circulation of water through the sand deposit of which our rocks are made dissolves part of the grains, and the silica taken up is redeposited on others. I cannot explain the chemical reaction that produces this deposition, but that it takes place in the rock during some period of its history is certain. I exhibit a quartzite pebble taken from the Triassic sandstone at Stanlow Point, which, as can be easily seen, was at one time worn perfectly smooth by attrition and long-continued wear, for the quartzite is very hard. Upon this worn surface you will see spangles and facets which reflect the light, and on closer inspection it will be evident that they are crystals of quartz that have been deposited upon the surface of the worn pebble after it became finally enclosed in the rock.