PETROGRAPHICAL NOTES. 79 



In some of the coarser-grained fragments they occur in long 

 lath-shaped crystals exibiting well marked twin lamellae and quite 

 unaltered. In finer-grained fragments their shapes are greatly or 

 entirely obliterated by innumerable matted minute prisms of pale 

 green hornblende, representing the original augite of the base, novr 

 entirely metamorphosed. It is not improbable that some of the 

 substance of the felspar has contributed towards the formation of 

 this variety of hornblende. Some of the rock fragments are almost 

 obscured by iron ores in small grains as is often seen in a fine-grained 

 basalt, or else finely disseminated ilmenite is entirely altered to 

 leucoxene. Considerable quantities of epidote and chlorite occur as 

 usual. Here and there may be seen larger crystals of pleochroic 

 uralite, indicating the previous existence of porphyritic augites. 

 Sometimes they are twinned and their substance is compact, but 

 often they become fibrous and are frayed and torn along the direction 

 of bedding, showing that although the clastic structure is original the 

 rock has nevertheless been subjected to considerable motion since its 

 consolidation. 



The clastic or brecciated nature of the rock T y ¥ is very con- 

 spicuous in the hand-specimen, but is not visible in the microscopic 

 section on account of the large size of the fragments. It shows 

 under the microscope a very fine-grained basaltic groundmass through 

 which are scattered some narrow porphyritic felspars, as much as 

 half a millimetre long and arranged along the direction of flow, and 

 Uralite with the shape lar g e beautiful pleochroic uralites, some of 

 of augite. which are over a millimetre in diameter. 



These reproduce accurately the shape of the original augite and 

 even such details as partial corrosion by the fluid magma, and at 

 the same time indicate the change in mineralogical composition 

 mentioned by Rosenbusch and other authors as frequently accom- 

 panying the uralitisation of augite : the large crystals are some- 

 times surrounded by a fringe of calcite and chlorite, and in the 

 interior of their mass patches of chlorite with epidote crystals are 



( 79 ) 



