AND SPHEROIDAL STRUCTURE. 149
hut on closer examination it becomes evident that this apparent division is caused by a change in the direction of the faces of the column, so that for a very short space, at this point, they are no longer vertical, but bend very slightly in or out. Nearly all these apparent bands curve a little upwards. Sometimes they correspond with actual cross joints; at others there may be very fine con- cealed cross joints; but in many there is not the slightest sign of any division at all. Still these bands indicate a tendency to division ; for at a short distance the columns become less regular, and the real cross joints (still curving upwards) more numerous, so that the lava is first divided into irregular rectangular blocks, with curved upper and under surfaces, and then broken by irregular curving joints, which in a few cases approach the curvitabular structure already described ; so that in one part the columnar, in another the curvitabular structure is dominant.
To proceed to the spheroidal structure. Professor J. Thomson regards it as the result of a process of exfoliation due to the action of the weather on a tolerably regular-shaped homogeneous mass. Though undoubtedly cuboidal blocks of rock have, for obvious reasons, a tendency to weather into rough spheroids, and though further action of the weather might occasionally produce concentric exfoliation in such spheroids, yet I trust to show that this explana- tion is wholly inadequate in the present case, although doubtless the weather has great effect in developing the structure. The ex- amples which I am about to mention will, I think, establish these two propositions : —
(1) Spheroidal structure is to be seen in rocks which are not homogeneous and are not at all cuboidal in form.
(2) Spheroids may be found in columnar rock which have evi- dently formed inside a prism, the exterior of which was not broken by joints.
Spheroidal structure has been observed in plaster on a wall*. A very fine example of it in bedded shale is figured by Mr. Jukes in his * Manual of Geology 'f. I have seen it well developed (of an ellipsoidal form) in a lenticular fragment of shale caught up in basalt on the Fifeshire coast, near Elic. A very fine instance of it may be seen in volcanic ash near the village of Santa Lucia (Yalle do Cordevole, Italian Tyrol). Here it is so conspicuous that the rock at a short distance might be readily mistaken for a decomposing basalt. Instances of it can also be found in the agglomeratic ash of the Binns, Burntisland (Fife). The annexed diagram (fig. 10) will show that here the structure is wholly independent of the form of the rock. Sometimes also it occurs where a distinct stratification may be observed, through the planes of which it cuts.
Again, perlitic J obsidian and pitchstone is a true case of sphe-
- Geol. Mag. vol. viii. p. 333. t Manual of Geology, p. 311 (ed. Geikie).
| I distinguish, of course, perlitic from sph;ernlitic structure. The best spe- cimens which have come under my own notice have been from Hungary : for the opportunity of examining a very interesting example I am indebted to Mr. Judd'e kindness.
