AND SPHEROIDAL STRUCTURE. 



145 



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- 



