PETR0GKAPH1CAL NOTES. 89 



2. Tuffs and Breccias. 



The tuffs associated with the rhyolites are easily distinguishable 

 from them in the field mainly by reason of their lighter colours, their 

 mode of weathering, and their undoubtedly bedded aspect. In many 

 cases the larger fragments and lapilli they contain stand out con- 

 spicuously on the weathered surface of the rock. Under the microscope 

 the matrix is hardly to be distinguished from the groundmass of some 

 of the rhyolites, and they are evidently formed of the same material, 

 blown into dust and subsequently consolidated by pressure. The pre- 

 sence of fragments and lapilli of quartz and other minerals as well as 

 fragments derived from previously consolidated flows is sufficient proof 

 of their clastic origin. One of the tuffs, No. 1 1500, contains numerous 

 flakes of brown mica ; this is the only instance in which i have found 

 this mineral associated with any of the Malani rhyolites. 



The breccias usually occi r in lenticular bands intercalated with the 

 lava flows. They consist of irregularly shaped angular fragments of 

 various sizes derived from previously consolidated flows imbedded 

 in a more or less opaque paste. The condition of the fragments 

 affords further argument in support of the contention that the crys- 

 talline structures developed in the groundmass were original, that is, 

 formed at the time of consolidation, and are not due to subsequent 

 processes of devitrification ; for in one and the same hand specimen of 

 breccia fragments may be found which exhibit each of the struc- 

 tures I have described, "quartz mosaic", microcrystalline and flow- 

 structure, which must have been developed in them before they 

 were broken up and included in the breccias. In one case, however, 

 in a bed of breccia extending over a considerable area in the low 

 ground west of the group of hills near Nagona, the included fragments 

 possess a structure, which may be due to devitrification developed in 

 them after they were included in the matrix. The fragments are mostly 



( 89 ) 



