THE TRAP DYKES OF THE ORKNEYS. 885 



common with the camptonites. Here, however, it does not surround augite, nor does 

 it form those frayed-out fringes which he describes. 



Classification of the Camptonites. — Rosenbusch (XV., vol. ii. p. 540) has proposed to 

 subdivide this series of rocks, according to the porphyritic minerals they contain, into 

 hornblende-camptonites with olivine, augite, and hornblende ; biotite-camptonites with 

 olivine, augite, and biotite ; biotite-hornblende-camptonites with olivine, augite, biotite, 

 and hornblende ; but, if applied to the Orkney dykes, this would result in a very 

 artificial grouping. Many of them contain no phenocrysts, except occasional olivine. 

 Others contain only olivine and augite. Biotite is very wide-spread, and perhaps universal, 

 but in such small amount, and always so inconspicuous, as to be best regarded as an 

 accessory. In the groundmass both hornblende and augite usually occur, but in a few 

 slides, which are perfectly fresh, only hornblende is to be seen. The same rock may 

 contain both minerals in the groundmass at the edges, while only hornblende is present 

 at the centre {e.g., Galton). 



In the literature of this group many descriptions are given of rocks which contain 

 only hornblende in the groundmass, mixed with green chlorite, which is regarded as 

 secondary after hornblende. I very much doubt whether this is always the case. The 

 augite decomposes so much more readily than the amphibole, that it may be quite 

 decomposed while the other is fresh, and, owing to the frequent parallel growths, the 

 outlines may be those of hornblende, and the crystal appear to be a hornblende, which 

 at one part has weathered into chlorite. In many such cases sections of fresher material 

 from the same dyke have shown the presence of augite, and, after a careful study, the 

 conviction has been forced upon me that it is only very rarely it can be concluded that 

 augite is entirely absent from the groundmass. To subdivide the rocks into camp- 

 tonites with hornblende and plagioclase in the groundmass, and augite-camptonites which 

 contain augite in addition, would lead us in some dykes to give different specific names 

 to the central and marginal parts of the same mass. These two minerals, in fact, are 

 so closely related, both as phenocrysts and as constituents of the groundmass, that in 

 the Orkney dykes such a subdivision would be highly unnatural. 



What might be called a pure augite-camptonite, were the term not already employed 

 in a different signification, is shown in a section from a loose block of trap I picked up on 

 the beach a little to the south of Burness, Firth. The original dyke must be concealed 

 by the gravel of the shore. The section shows phenocrysts only of olivine altered into 

 serpentine, and consists of a very pale violet augite, quite idiomorphic, in short 

 stout prisms, which are often in radiating groups and perfectly fresh. Magnetite, 

 ilmenite, and scales of biotite are scattered through the slide. There is no hornblende. 

 Plagioclase felspar in well-formed crystals occupies the spaces between the augites, and 

 forms occasional ocelli, with calcite in the centre. The structure is that of the camp- 

 tonites, but their most characteristic mineral, brown hornblende, is quite absent. The 

 general appearance of the slide will be seen from the photograph (PL III. fig. 1). 



In the Bay of Binniaro, a little south of the locality just mentioned, occurs a group 



