BY W. N. BENSON. 603 



Pyroclastic Rocks. 



It is in this division of the igneous rocks, that the greatest diffi- 

 culty arises, for they range from types scarcely separable 

 from the normal massive rocks to those which are clearly tuffs, 

 and agglomerates that closely resemble conglomerates. They 

 also have a wide range of composition, corresponding to 

 the range of variation in the massive rocks, and may include 

 fragments of all the known types of massive rocks, as well as 

 others, which have not been found forming separate masses; they 

 may contain also fragments of all types of sedimentary rock in the 

 district. This naturally makes a logical sequence in the description 

 of these rocks almost an impossibility. It seems, therefore, best to 

 divide between pyroclastic masses that are referable to one igneous 

 type only, and those of a mixed composition. The latter can 

 be divided according to their coarseness of fabric. Of these, some 

 of the most coarsely fragmental types have rounded inclusions re- 

 sembling boulders in conglomerates. 



Of the rocks composed of a single type of material, we may, 

 perhaps, take as an instance the augite-porphyrite breccia already 

 described. It certainly is a passage-rock of a nature between mas- 

 sive and fragmental. A similar passage-rock is the spilite-breccia, 

 which lies on the margin of the spilite-dolerite mass at Tintinhull. 

 This rock consists of more or less fragmental phenocrysts of albite, 

 set in a very fine matrix of the same character as that of the 

 adjacent porphyritic spilite(1171). The brecciated quartz-kerato- 

 phyres should also be mentioned here. Types occur, particularly 

 in Portion 183, Nemingha, on West Gap Hill, which are even more 

 fragmental than the highly strained keratophyres mentioned above. 

 They have a grey quartzite-like appearance, and show small crys- 

 tals of felspar, and have a more or less well marked bending. They 

 consist of crystals of albite, irregular corroded quartz-grains, and 

 sometimes fragments of a very fine-grained traehytie keratophyre, 

 and aggregates of quartz and magnetite, drawn out into the general 

 direction of the banding. The groundmass consists of the same 

 material very finely divided. It is possible that these are flow- 

 breccias (1052, 1125). 



