S. J. Loive — Rock Differ ent'mtion. 347 



Apatite in long prisms of varying stoutness is sometimes abundant, in 

 other instances rare. The constitution of this variety of the Lurcomhe 

 rock-mass corresponds with that called camptonite (Rosenbusch) or 

 hornblende lamprophj're (Harker). 



The varieties of the rock classed as grades II and III are those in 

 which augite is associated with the hornblende. In grade II both 

 these minerals are found in idiomorphic form, sometimes in about 

 equal proportions, though in most instances obtained the augite 

 predominates, and these merge into grade III, where very little 

 hornblende is met with, and that almost entirely attached peripherally 

 to the augite, a way in which it also occurs in grade II. 



In these varieties the hornblende is exactly similar to that of 

 grade I, though not in such well-developed crystals, and varies in 

 relative quantity to the augite from a slightly preponderating pro- 

 portion to insignificant attachments to the augite, with occasional 

 patches contained within the augite crystals. The augite is in well- 

 formed idiomorphic crystals giving varying flesh tints with rotation, 

 and brilliant double-refringence colours, showing occasionally^ hour- 

 glass structure and other zonal markings. Twinning also occurs. 

 This mineral is in a better state of preservation generally than the 

 hornblende, though chloritization has frequently^ taken place in patches 

 and lines within the crystals, with the usual accompaniment of calcite, 

 its related decomposition product. 



The felspar diminishes in relative amounts with the increase and 

 predominance of the augite. It however maintains the characteristics 

 already described, there being still platy, lath-shaped, and minute 

 curved forms ; the larger irregularly shaped plates are much micaized 

 and occasionally have partial surroundings of fresher material and of 

 more acid constitution. In these two grades of the rock a difference 

 is noticeable frequently, in the feebly translucent interstitial material 

 before mentioned. Clear spaces are to be seen containing an almost 

 isotropic mineral, detected by faint lines of cleavage in the shadowy 

 illumination got by rotation. This additional secondary mineral was 

 doubtfully considered to be analcite, but by Dr. Flett's kindly assistance 

 a certainty in the matter has been established. In no instance has 

 the analcite definite boundaries, but it merges into the surrounding 

 material, the apparent bye-products of its formation. Calcite is 

 always associated with the analcite bordering and veining it, and 

 chlorite is occasionally seen partially surrounding it. There is also 

 an instance of analcite with its usual border and veins of calcite grains 

 occurring within an augite crystal. 



Besides the mica which mosaics the felspar plates some small 

 crystals of a coloured variety are occasionally seen in the interstitial 

 material. The accessories are the same as noted in grade I. These 

 varieties of the rock-mass, distinguished as grades II and III, thus 

 have a mineral constitution to which petrographical nomenclature has 

 ascribed the term Teschenites. 



The Lurcombe mass presents still another variation, which follows 

 the direction of differentiation indicated by those already described. 

 This variety, distinguished as grade IV, contains no hornblende, the 

 only original ferro-magnesian being augite, which assumes a sub-ophitic 



