Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/217

Rh AND SPHEROIDAL STRUCTURE.

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regular than their distant appearance would lead one to infer, as they vary from irregular quadrangular to hexagonal, the majority appearing to be pentagonal. They are often as much as 6 feet in diameter. The great cross joints cut the columns at distances from 2 to 5 feet apart ; and the whole mass is traversed by a fissile struc- ture curiously resembling that of a roughly cleaved slate, parallel to these joints. As the outward dip of these planes is some 20°, it produces a most unpleasant idea of instability upon the mind ; for the columns look as if the least disturbance would cause them to slip forward. At the southern end of the cliff the columns are less distinct ; and here the curve of the divisional planes makes a much higher angle with the horizon than elsewhere, becoming nearly vertical (see figs. 3 & 4).

From the above descriptions it will be evident that the fissile structure in these rocks cannot be a true cleavage ; its explanation will be attempted presently.

(b) Tabular Structure. — In many igneous rocks the horizontal joints are so regularly and strongly defined as to produce a tabular or bedded structure. Instances of this may be seen in many granites, as, for example, at Pardenick Point, near the Land's End ; in dolerite, in an intrusive mass under " the Castle," Burntis- land (Fife) ; in the gabbro of the Gimlet rock, Pwlhelli, and of the Cuchullin Hills ; and, though less perfectly, in some of the " traps " in the Lake-district. A case which I have more than once observed seems to suggest some connexion between this and the platy struc- ture described above. An excellent example of it may be found in a quarry on the Gross Weilberg (Siebengcbirge) (fig. 5). Here a mass of

Fig. 5. — Columns Sfc. (Gross Weilberg).

A. Trachytic tuff. B. Closely jointed rude columns. C. Bent columns. D. Upper stage of quarry with good columns. E. Lower stage. F, Platy structure. G. The same in rude columns.

basalt is intruded into a trachytic tuff. The exterior of the mass at the top is for the space of 3 or 4 feet divided into rude quadran-