A. L. Du Toit—Pipe-Amygdaloids. 15 
The lavas, which are buried almost horizontally here, are in places 
traversed by nearly vertical cylinders of amygdaloidal rock from 
one to six inches in diameter and with considerable length. Cross- 
sections are circular, elliptical, crescentiform, or a little irregular 
in outline; the spacing of these vesicular portions is usually very 
irregular. 
The cylinder walls are clearly defined from the enclosing compact 
diabase, owing to the occurrence on their peripheries of closely-set 
vesicles elongated in a vertical direction; apart from their vesicular 
character, there seems to be no difference in the nature of the material 
composing them (Fig. 2). 
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Fic. 2.—Vesicle cylinders in diabasic lava from Barkly West. (a) longitudinal 
fracture along, (c) transverse fracture across, a cylinder. In (b) a row of vesicles haye 
coalesced to form a ‘ pipe-amygdale.’ (Drawn about 4 nat. size.) 
At the town of Barkly West a bed of this diabase rests upon an 
earlier extruded sheet of amygdaloid, and the lower eight or ten feet 
of the compact rock is traversed by many cylinders of vesicles. 
In some of the cylinders certain numbers of the bubbles have 
coalesced to form irregular pitted cavities, and in one case a long 
pipe-amygdale, about two-thirds of an inch in diameter, has been 
produced; ordinary pipe-amygdales are developed in places at the 
extreme base of the flow. 
The cylinders may terminate sharply upwards in the form of 
a cone-shaped mass of vesicles, or the bubbles may become diffused 
through the diabase. 
The vesicular portions of the rock are more readily attacked by 
weathering, and surfaces of the diabase exhibit shallow pits. At 
Niekerk’s Rush, on the Vaal River, these hollows are so regularly 
spaced in the diabase along the river bank as to have led many 
persons to imagine them to be the footprints of prehistoric animals. 
The areas are D-shaped in plan and similarly oriented; the convex 
side apparently indicates the direction of a movement in the lava 
during its solidification. 
Structures having a very great resemblance to, and in some cases 
apparently identical with, these have been described from the 
