78 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



least definite, the rock is traversed by vertical joints, somewhat more regular and close- 

 set than those in the dolerites, by the intersection of which it is separated into rude 

 quadrangular or polygonal columns. The true prismatic structure is shown in two chief 

 forms, (a) The rock is divided into close-fitting parallel, usually six-sided, columns ; 

 the number of sides varying, however, from three up to nine. The columns run the 

 whole thickness of the bed, and vary thus from 8 or 10 to 40 or even 80 feet in length. 

 They are segmented by cross joints which sometimes, as at the Giant's Causeway, take 

 the ball and socket form. Occasionally they are curved, as at the well-known Clam-shell 

 cave of Stafla. (b) The prisms are much smaller, and diverge in wavy groups crowded 

 confusedly over each other, but with a general tendency upwards. This starch-like 

 aggregation may be observed superposed directly upon the more regular columnar form, 

 as at the Giant's Causeway and also at Stafla. 



3. It may often be noticed that, even where the basalt is most perfectly prismatic, it 

 presents a cellular and even slaggy structure at the bottom. The rock that forms the 

 Giant's Causeway, for instance, is distinctly vesicular, the vesicles being drawn out in a 

 general E. and W. direction. The beautifully columnar bed of Stafla is likewise slaggy 

 and amygdaloidal for a foot or so upwards from its base, and portions of this lower layer 

 have here and there been caught up and involved in the more compact material above 

 it. Even the bottom of the confusedly prismatic bed above the columnar one on that 

 island also presents a cellular texture. A similar rock at Ardtun, in Mull, passes 

 upward into a rugged slag and confused mass of basalt blocks, over which the leaf-beds 

 lie. At Loch-na-Mna, in the island of Eigg, one of the basalts presents in places a 

 remarkable streaky structure, due, doubtless, to the arrangement of its component 

 materials during the flow of the still molten rock. 



Amygdaloidal structure is more or less well developed throughout the whole series 

 of basalts. But it is especially marked in certain abundant sheets, which, for the sake of 

 distinction, are called amygdaloids. These beds, which form a considerable proportion of 

 the materials of every one of the plateaux, are distinguished by the abundance and large 

 size of their vesicles. In some places, these cavities occupy at least as much of the 

 rock as the solid matrix in which they lie. They have generally been filled up with 

 some infiltrated mineral — calcite, chalcedony, zeolites, &c. The amygdules of the west of 

 Skye and of Antrim have long been noted for their zeolitic enclosures. As a consequence 

 of their cellular texture and the action of infiltrating water upon them, these amygdaloidal 

 beds are always more or less decomposed. Their dull, lumpy, amorphous beds contrast 

 well with the prismatic sheets above and below them, and as they crumble down they 

 are apt to be covered over with vegetation. Hence, on a sea-cliff or escarpment, the green 

 declivities between the prominent columnar basalts usually mark the place of these 

 less durable bands. 



Exceedingly slag-like lavas are to be seen among the amygdaloids, immediately 

 preceded and followed by beds of compact black basalt with few or no vesicles. From 

 the manner in which such rocks yield to the weather, they often assume a singularly 



