Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/219

Rh AND SPHEROIDAL STRUCTURE.

147

horizontal curving joints with their convexities upward ; so that the whole is divided into a series of plano-convex, meniscoid, or concavo-convex blocks, in length perhaps about 2 to 4 feet, and in thickness 4 to 12 inches (fig. 6). This structure continues with but slight indications of any tendency to vertical master joints for some 35 feet ; and then the basalt for the remaining space (perhaps about 4 yards) becomes rather suddenly rudely columnar*. How much has been removed by denudation from the top of the flow it is impossible to say. At the end of the spur there is evidence that its surface was uneven and a ridge already defined when the basalt flowed; for the tabular structure curves in both the longitudinal and transverse sections, so as to remain symmetrical with the surface of the ground. It is therefore evident that it is con- nected with the form of the surface of the rock. Close to the station of Eougeac (railway from Le Puy to Arvant) I saw a similar structure in a mass of basalt. A very rough sketch of this in- teresting section (jotted down from the railway -carriage window) is given below (fig. 7).

Fig. 7. — Section at Eougeac.

A B C D

A. Basalt. B. Stratified volcanic ash. C. Aggloineratic volcanic ash. D. Basalt, with curvitabular structure.

Here also the structure is affected by the form of the bounding surface ; but in this case the convexities of the curves are turned downward.

(d) Cup-and-ball Structure. — The next structure to be noticed is that form of cross-jointing of columnar rock which goes by the name of cup-and-ball structure. Usually the columns are divided across, at variable distances, by plane cross joints at right angles to their axes. These sometimes, as in the Gross Oelberg and Weilberg (Siebengebirge), and at Murat (De'partement de Cantal, Auvergne), are as much as 15 or 20 feet apart ; but commonly they are not more than a yard, and sometimes less. Occasionally, however, the joint- surface is not a plane, but more or less curved, so that the convex extremity of one segment fits into the concave extremity of the next. The form of this curve varies : sometimes its curvature is but slight, and it is continued down to the sides of the prism ; at others the curvature is more marked, the outline of the ball being more nearly circular, and is not continued to the sides of the prism, a flat space

very regular prisms of five or six sides, which exfoliate by decomposition in slaty lamina; at right angles to their axes. I do not remember to have seen this structure conspicuous in the part which I examined.
 * Mr. Scrope (p. 106) describes this basalt as separated at some points into