﻿442 ME. J. B. SCEIVENOE ON THE [A-Ug. T 9 IO i 



Some pyrite was visible. Under the microscope, by reflected light, it was seen 

 that the rock is largely composed of pyrite and an opaque white substance. Blue 

 corundum occurs in veins, and there is a little brown mica in small flakes. 

 The platy mineral, which is abundant, was separated and proved to be white 

 mica with a wide axial angle. A few grains of tourmaline were seen. The 

 percentage of corundum in this rock was determined as 16'6. 



A still more remarkable rock was found in the head-waters of the Eiver 

 Johann (PI. XXX, fig. 6). A section shows discs and rings of corundum, set in 

 a base that consists entirely of white mica. They are sometimes white, some- 

 times black with an included dust, and sometimes showing concentric structure 

 by means of this dust, which also imparts to some of the discs a spotted appear- 

 ance. It was found, by separating the corundum, that the discs and rings are 

 composed of small shapeless grains. 



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Dark rocks with corundum and tourmaline. 

 The colour of worn specimens is generally deep blue-black. The majority 

 of specimens show cavities or bodies, measuring about 6 millimetres, which 

 suggest vesicles or amygdaloids in lava. In some cases the rock is compact. 

 The specific gravity of a compact specimen proved to-be 301. All these rocks 

 are very hard and fine-grained. In nearly all of them a dense black dust is 

 observable, which sometimes is so thick as to make the slide opaque. This was 

 proved, by treating the crushed rock, to be, for the most part at any rate, some 

 form of carbon. 



(a) The Eed hills type-rock, in situ. — In a hand-specimen this rock is 

 dull black. The grain is fine, but the rock is very rough to the touch. Through- 

 out it are cavities about 6 millimetres across, some round, some elongated, some 

 irregular. These are sometimes empty, sometimes partly filled or lined with 

 minute lustrous tourmaline crystals, and they sometimes show a faint pale 

 border outside these crystals. The rock is very hard and heavy, and might 

 easily be mistaken at first sight for a vesicular lava. 



In section the rock is seen to consist of tourmaline, corundum, and black 

 carbonaceous dust. The grain varies slightly, and when coarsest there is little 

 or no structure to be seen. In the fine-grained rock, however, the structure is 

 remarkable. With powerful illumination it is seen that there are present 

 numerous round and oval bodies, and also large cavities, which are the cavities 

 seen in the hand-specimen. The smaller bodies are composed of corundum 

 alone, while the larger have a shell of corundum and a core of tourmaline 

 grains. In one case the core consists of a single grain of tourmaline. 



The corundum shells are made up of small shapeless grains (PI. XXXI, 

 fig. 3). The large cavities are of a similar structure. Generally there is an 

 outer border of corundum grains, and an inner border of tourmaline. 

 PI. XXXI, fig. 4 shows an exceptional case, where the border is composed of a 

 double band of corundum grains with tourmaline and corundum between these 

 two bands, and tourmaline on the inside of the inner band. 



Apart from these structures, the corundum occurs as irregular and vein-like 

 masses. 



The tourmaline rarely forms well-defined crystals, and in this case is brown. 

 The corundum is sometimes pale blue. 



It may be mentioned here that the largest tourmaline grains measured in 

 any of these tourmaline-corundum rocks were only 037 mm. in greatest width. 



In some detrital specimens large bodies are found of the same size and 

 shape as the cavities in the Eedhills type-rock. They are solid, and consist of 

 tourmaline and corundum, which, generally speaking, are arranged in such a 

 way as to suggest a rough concentric structure, with the corundum more 

 abundant near the outside of the bodies than in the centre. In the Eedhills 

 type-rock there is no reason to suppose that the cavities ever contained a solid 

 core of tourmaline, or of corundum and tourmaline, although it is probable 

 that the original structures replaced by these minerals were solid. 



