MELAPHYHES AND KKLSITIS OF CAKADOC. 531) 



road to Comley. (T)olcrite.) — A greenish-grey holocrystalline rock of 

 rather coarse texture, mainly consisting of dark-green augite and 

 greyish felspar. 



Under the microscope the rock is seen to consist of crystals of 

 perfectly unaltered augite, labradorite often partly converted into 

 kaolin, magnetite, and interstitial patches of chlorite. The augite 

 crystals, as a rule, appear to range from more than three millimetres 

 to about one millimetre in length. The crystals of labradorite are 

 frequently of somewhat larger dimensions. 



III. Conclusions witjl kegakd to the MELAiMiYiiLS. 



The foregoing descriptions show that, within a very limited area, 

 the melaphyres of Caradoc differ considerably in texture and in 

 structure. Some have once been basalt-glass or andesite-glass, such 

 being the superficial portions of a lava-stream ; others have pos- 

 sessed a certain amount of interstitial glass which has subsequently 

 been rendered more or less opaque by the development of magnetite, 

 while, at times, it ai)pears to have been converted into a substance 

 possibly allied to palagonite. 



In some of these rocks the crystalline texture is very fine (pilo- 

 taxitic), while in the case of the dolerite from Little Oaradoc it is 

 comparatively coarse. Furthermore, near the summit of Caradoc 

 we have a basalt-tuff* or andesite-tuft". 



These rocks are spoken of as altered basalt or andesite, since any 

 pyroxene or olivine which they may once have contained is in most 

 cases so completely replaced by alteration-products that it is im- 

 possible to define their original mineral constitution with precision. 

 This is also partly due to the allotriomorphous character of those 

 minerals which have undergone decomposition. The felspars and 

 magnetite are, as a rule, the only original constituents which remain 

 unaltered, and these at times have suffered very considerable change. 



The term melaphyre, as indicating an altered basalt or andesite, 

 seems perfectly applicable to theee old lavas. 



The dolerite of Little Caradoc differs from these rocks, in that the 

 augite remains perfectly fresh, or is only altered along minute fissures, 

 and the felspars are more or less turbid and altered, while in the 

 lavas of Caradoc proper it is the pyroxenic constituent w hich has 

 undergone decomposition, but the felspars remain fresh and, as a 

 rule, unchanged. 



Whether the dolerite of Little Caradoc may be regarded as a 

 volcanic neck or plug, from which the basic lavas lying to the south- 

 west of it emanated, is a point which field-work can alone demon- 

 strate or disprove, but, taking into consideration the gradations of 

 texture which these rocks jjresent at different levels, such a sup- 

 position comes within the range of possibility. 



