The

directions of the columns in the several tiers of prisms. wondrous regularity of the columns and their small size are particularly striking in this exposure, viz. they are nearly all from three inches to five inches long, and little more than half an inch thick (see Fig. 2, natural size). They are by no means constantly hexagonal, but vary from three- to seven-sided. In all, however, the faces are clean and well defined, and the angles sharp. The rock material of which they are composed is a brittle sandstone, so hard that the columns clink when struck against one another, but loose and porous in appearance when examined on a broken surface. On such fractures it is seen to be made up of small irregular quartzose grains, which look as though fused together at their edges, but with vacant spaces as great as themselves left abundantly amongst them. Towards their exterior the columns are more compact, and upon the joint surfaces themselves the quartz grains are more or less fused together; indeed, we may often see here a continuous layer of vitreous quartz over a small area, where the outlines of the component grains are only indicated, if at all, by such a granular appearance as is seen in a compact quartzite. It is as though each prism had been half fused in a hot mould, so that the clean surfaces and sharp angles were most altered by the heat, and the interior was less so.

There is no such special structure on the joint surfaces of the ordinary Quader Sandstein. Comparing the fractured surface of one of these columns with rock specimens from the ordinary Quader Sandstein, I cannot detect any very striking differences between them. The columnar rock is even of looser build, internally, than usual; but the whole column is harder and less easily crumbled than the unaltered rock, this being especially the case with the surface layer.

A microscopic examination and comparison of prepared thin sections of the two kinds of rock, the altered prismatic and the ordinary sandstone, does not yield us any special characteristics of distinction. Professor Bonney, who has kindly examined the slides, writes concerning the columns, that they "consist of angular to rather rounded quartz grains, with one or two which may be decomposed felspar, cemented by a rather abundant dark substance (quære if this be not in part discolouration from powder used in preparing the slide, as the texture is rather open). In the quartz many minute inclosures, almost always less than 001", appearing when highly magnified of a pale purplish colour, and in form rather like irregular tubes. In one or two there was an appearance which might indicate a very minute bubble, but in most cases there was no sign of this. Very little difference between the two slides, except that the cementing substance in that from the column is paler, and the quartz grains look a little cracked."

We may next notice the well-marked layers in which the prisms are arranged. This is in no way connected with stratification lines of deposition, but is a superinduced joint-structure such as occurs not unfrequently in prismatic basalt masses, e.g. in the great basalt

1

quarry above Forst, near Durkheim. It is the "Tabular" structure of Professor Bonney. In these altered prismatic Saxon sandstones this structure is unusually regular, dividing the rock into zones about 4 inches in thickness.

According to A. von Gutbier, an irregular line of clay ironstone 4 to 5 inches thick, and next a zone of "reddish green steinmark,” intervened between the sandstone columns and the basalt.

Next, to inquire into the mode of origin of these prisms and their associated structures. The production of columnar structure by contraction during cooling or drying is now very well understood; being well illustrated by the structure of dry starch on the one hand, and in the artificial formation of columns in the sandstones of furnaces on the other. The evidence of the former presence of heat in close proximity with the Gorischstein prisms is obvious enough, for the outermost sandstone columns were almost in contact with the liquid lava mass. But whether the heat was also the immediate cause of the prism of the quartz grains, as above described, on the surfaces of the columns, is not quite so clear; though the fact of its special association with the prismatic structure must not be overlooked. There is no evident reason why the deposition of silica in the wet way should go on over the prismatic joints differently from that over the ordinary rock joints, except during that special period of cooling when the columnar joints were in course of formation. I am therefore disposed to refer this alteration of the superficial layer of the prisms to the later stages of the volcanic period, when such joints would have served for the passage of heated water or of steam to the surface.

Note by the Rev. Professor Bonney on the origin of the curved arrangement of the prisms.

The rock in order to break requires (1) that its temperature should be considerably elevated; (2) that this should be again gradually lowered.

A surface of uniform temperature is a surface of

uniform tension.

Now if A B C be a heated mass, losing heat principally from A B, a rock at surface, the surface of uniform tension lies parallel to A B.

C

Again, if DEF be a mass in contact with an igneous rock K at some depth, the main loss of heat will be laterally, and so the surface of uniform tenison be vertical and the columns horizontal.

D

F->

At the top, then, the loss of heat will be mainly from the top; deep down mainly laterally. Thus the surfaces of equal tension will descend in the mass radially, the lateral loss producing no effect at first and vice versa. So that I take it just at first the columns should lie pretty nearly parallel with the basalt,

E

1 Quite a distinct thing from the bedding of successive lava flows. See Prof. T. G. Bonney, Quart. Journ. Geol. Soc. vol. xxxii. p. 145.

and at last become perpendicular; of course the top layers will be formed long before the bottom.-T.G.B.

In a paper on "Columnar, Fissile, and Spheroidal Structure," read before the Geological Society (Q. J. G. S. vol. xxxii. p. 141), the Rev. Prof. Bonney has given a list of examples of columnar structure in non-igneous rocks. Amongst these the altered clay bed in Tideswell Dale1 most nearly resembles our Gorischstein prismatic sandstones. But the Yorkshire clay prisms are from 1 to 6 inches thick and 8 to 9 feet long (Mello), and the columns themselves are curved, as is common in basalts, etc.

In addition to the examples mentioned in the paper above referred to, Prof. Bonney now tells me of a columnar sandstone underlying basalt from Johnsdorf, near Zittau, Saxony; ironstone, columnar from burning, exhibiting bent, wavy and radiating columns, about

" diameter, from South Wales; and columnar sandstone from furnaces in Wales, Staffordshire, and Russia-all of which may be seen in the Museum of University College, London.

Another good example of this structure was observed by my friend Mr. A. F. Griffith, of Christ's College, last year, near Clermont, Auvergne. It occurs in a pale marl under the old lava flow of Graveneine near its extremity, about three-quarters of a mile from Clermont on the Issoire road. Here a quarry has been opened so as to expose some 12 feet of the lava; beneath this is a zone of porcellanized material, irregularly jointed, and about two inches thick; and next below comes a layer of diminutive hardened columns 5 or 6 inches in thickness. The better prisms are about 4 inches long, and inch across; but others are not more than 11⁄2 inches long. Beyond the limits of the lava, the prismoidal clay is seen to pass into a pale freshwater marl. Mr. Scrope has described a similar case near St. Saturnin beneath the lava current of the Puy de la Vache.

III. FURTHER NOTES ON A COLLECTION OF FOSSIL SHELLS, ETC., FROM SUMATRA (OBTAINED BY M. VERBEEK, DIRECTOR OF THE GEOLOGICAL SURVEY OF THE WEST COAST, SUMATRA). PART II.3 By HENRY WOODWARD, LL.D., F.R.S., etc.;

A

of the British Museum.

(PLATE XI.)

MONG the Conchifera transmitted by M. Verbeek, is a specimen of Cyrena. The shell is certainly from a very modern formation, as it retains its translucency and traces of a pale buff colour externally.

19. Cyrena sinuosa, Deshayes. Pl. XI. Fig. 1.

Mr. Edgar Smith, after comparing this specimen with recent Cyrena in the collection, observes, "The outline of this valve is

Described by the Rev. J. M. Mello, Quart. Journ. Geol. Soc. vol. xxvi. p. 701, and GEOL. MAG. Vol. VII. p. 520.

2 Volcanos of Central France, p. 92.

3 Continued from the September Number, p. 393.