156 Proceedings of the Royal Physical Society. 



regarded in the aggregate as a dyke, and which cut through, 

 and harden, a mass of Lower Carboniferous tuff. The chief 

 feature noticeable in the field, besides the irregularity- 

 referred to, is a laminated or platy structure, which runs 

 parallel to the bounding surfaces of the mass, and which 

 suggests the result of weathering upon a banding of alter- 

 nate harder and softer zones, due originally to fluxion 

 structure. There is a good figure of the mass in question 

 given in Geikie's "Ancient Volcanoes of Great Britain," 

 page 409, from a photograph by Mr Robert Lunn. 



In hand specimens the rock is heavy, has a rusty surface 

 where weathered, and shows a dark-coloured, close-grained, 

 and compact ground mass, through which are scattered 

 crystals, also of a dark colour, and with a very feeble lustre 

 on their faces of fracture. 



An examination of the rock under the microscope showed 

 that it consisted of Olivine, Augite, and iron ores, set in a 

 brownish, lithoidal paste, through which are scattered a few 

 minute lath-shaped crystals of a plagioclase felspar, which 

 appears to be Labradorite. On making a comparison of 

 this rock with the Haddington Limburgites described by 

 Dr Hatch, and with some typical specimens from the 

 Kaiserstuhl, the resemblance was found to be very close, 

 and were it not for the presence of the small quantity of 

 minute felspars, the rock might be regarded as a true 

 Limburgite. Its chief interest, however, lies in the fact that 

 it clearly forms an intermediate link between an ultra-basic 

 rock and the typical basalts which occur so abundantly as 

 dykes and other intrusive masses in the immediate neigh- 

 bourhood. 



When this paper was read a scheme of classification of 

 eruptive rocks was laid before the meeting, chiefly with the 

 object of inviting a discussion as to how these intermediate 

 lithological types of rock might be dealt with. The difficulty 

 is greatly felt by those who have to do with the position of 

 such rocks in museums where a wide range of lithological 

 types is represented, and where those in charge wish as much 

 as possible to simplify petrographical nomenclature, and to 

 avoid the introduction of new names. 



