712 MR. R. H. RASTALL ON THE GEOLOGY OF [Nov. I9H, 



pseudomorphs, and it was not found possible to determine its 

 original character with certainty. In some specimens it seems to 

 have been biotite, while in others the form of the pseudomorphs is 

 rather more suggestive of augite or hornblende. 



The ground-mass varies a good deal in different specimens : in 

 the freshest of all it consists of minute lath-shaped crystals of 

 felspar, with grains of magnetite and flakes of chlorite and biotite. 

 One variety shows a considerable amount of quartz with the 

 felspar. In some specimens the structure of the ground-mass is 

 distinctly andesitic, with well-marked flow-structure indicated by 

 the arrangement of the felspars; while in the quartz-bearing variety 

 the ground-mass is very like that of the porphyrites, for instance, 

 of Canisp or the Cheviot Hills, In the more altered varieties, 

 on the other hand, the minerals of the ground-mass have been 

 converted into an aggregate of carbonates and micaceous substances, 

 with iron ores. 



The vesicles contain a considerable variety of minerals, of which 

 the most important are pale-brown mica, chlorite, and epidote, 

 generally with some quartz, and occasionally tourmaline. 



With regard to the petrographical character and classification of 

 this rock, it is difficult to make any definite statement. Owing to 

 the high degree of alteration, as shown by the abundance of 

 carbonates, epidote, and chlorite, it is quite evident that chemical 

 analysis would be not only useless, but misleading. Judging from 

 the characters of such minerals as are still determinable, the rock 

 appears to have been originally either a lava or an intrusion of 

 intermediate composition. Some specimens might be described as 

 typically andesitic in character, and the vesicular structure which 

 is so notable and conspicuous affords strong presumptive evidence 

 in favour of extrusion, or at any rate of intrusion under a somewhat 

 thin covering of rock. On the other hand the form of the mass is 

 difficult to reconcile with the supposition of a lava-flow, and is 

 much more strongly suggestive of an intrusion. Owing to the high 

 grade of the subsequent dynamic metamorphism, it is impossible to 

 determine whether or not the surrounding sediments have under- 

 gone any contact-metamorphism. The question must therefore be 

 left an open one, and the rock maybe provisionally described 

 as porphyrite or andesite. 



VI. The Otir elite-Hocks of Waai Kloof. 



When they are followed towards the east, the general character 

 of the Malmesbury rocks is seen to remain very uniform : although, 

 owing to the narrowing of the outcrop, the lower grits become less 

 conspicuous. 



About 12 miles east of Worcester, the Coos Hiver cuts through 

 the mountains in a narrow gorge called Waai Kloof, and near the 

 mouth of this gorge is an exposure of a very interesting rock. The 

 prevailing rock of this district is a highly crushed and siicken- 

 sided grit of uniform character, and presenting few points of 

 interest ; but in a small cultivated field, forming an expansion of 



