Geological Society of London. 87 



also intersect the shales at frequent intervals. Within the pipes 

 there are unaltered nodules of the same dolerite. With regard to 

 the relation of the diamonds to the rock of the pij)es in which they 

 are found, the author stated that he thought it probable that the 

 latter was only the agent in bringing them to the surface, a large 

 proportion of the diamonds found consisting of fragments. At 

 the same time he remarked that each pipe furnished diamonds of a 

 different character from those found in other pipes. 



Discussion. — -Mr. Maskelyne complimented the author on the contribution 

 which he had made to geological science, by showing that the rock in which the 

 diamonds occur is confined to the pipes such as he had described. He was able, 

 from specimens which he had examined, to state positively that this certainly 

 igneous rock differed essentially from the dolerites and other igneous rocks in the 

 neighbourhood. He would not attempt to give it a name, whether Gabbro, 

 Euphotide, or any other designation. The materials, so far as they could be 

 determined in their present altered condition, Avere such as would not build up any 

 one of the known rocks. There were garnets and minerals resembling clinochlore, 

 smaragdite, phlogopite, diallage, and calcite, and apparently another mineral 

 entirely serpentinized. The diallage was the true diallage, viz. a mineral of 

 augitic type. The mineral resembling clinochlore was, he had proved, vermiculite, 

 and the phlogopite was not improbably Jeffreysite. The calcite had been thought 

 to have been derived from the decomposition of augitic silicates, but this he 

 doubted. One specimen, from a depth of 130 feet, exhibited a completely serpen- 

 tinous metamorphosis, which had involved one mineral as yet undetermined, but 

 probably enstatite. The absence of felspars in all the rocks but one was singular ; 

 in one specimen from Bultfontein there was, however, an abundance of kaolinite, 

 proving the original presence of a felspar. He thought that the fracture of the 

 diamonds might be due to the breaking up of the rocks in which they are now 

 found. A fact pointed out by the late Prof Rose, viz. that the octahedral faces of 

 diamonds became grooved after burning by triangular striations, was of great im- 

 portance in considering the changes these rock-fragments had undergone, as many 

 of the diamonds from the upper beds had their surface striated in this manner. It 

 would be a question of interest to observe whether those from the lower depths 

 were similarly marked and equally shattered. 



Mr. Forbes observed that, owing to his not having had the opportunity of 

 examining the rocks in question, he could not speak as to their nature ; but as he 

 looked upon the rock Gabbro as being in the main composed of triclinic felspar 

 with diallage, he could not regard any of the rock-specimens shown as entitled to 

 this name, especially as they, as a rule, were even devoid of felspar. The black 

 rock, found deepest, appeared to him to resemble one of the altered basalts in 

 which Zirkel had shown that the olivine was changed into a peculiar greenish 

 serpentine-like mineral. 



Mr. Dunn, in reply, stated that he had merely made use of the term Gabbro as 

 a provisional name. There could, he thought, be no doubt of the volcanic origin 

 of the rock ; and moreover, near Bultfontein and elsewhere, there were sheets and 

 dykes of dolerite, but quite different from the rocks in the pipes. Some of the 

 fragments in the matrix were fragments of shale, mica-schist, and hornblende, 

 which had probably been brought up into the pipes from below. 



The Chairman remarked on the wide application of the term Gabbro, which 

 could hardly be said to imply any distinct mineralogical character. Looking at 

 the question broadly, the facts seemed to be, that among nearly horizontal shales 

 there were patches of rock of approximately circular form, with the shale turned 

 up around their edges, and altered for a certain distance. The appearances were 

 therefore those of the extrusion of a heated or molten body by expansion from 

 below ; and possibly, as the author suggested, other traces of eruption had been 

 removed by subsequent denudation. He could not regard the rocks in the pipes 

 as in any way metamorphic, in the usual acceptation of the term. Though the 

 volcanic origin of those rocks might be accepted, he thought it more probable that 

 the diamonds were brought up from some subterranean metamorphosed rock, than 

 generated in the lava. 



