240 MESSRS. T. C. CANTRILL AND H. H. THOMAS ON THE [May I906, 



has been applied, more or less common in the amygdules of ancient 

 rocks which carry ferromagnesian minerals. 



Another variety of andesitic rock [E 4439] was observed to form 

 a thin band, only 8 feet thick. It is of a dark-green colour and 

 fine-grained texture, and occurs in the upper andesitic series 

 80 yards north of Llwyn-celyn. This rock has a specific gravity 

 of 2' 65. Under the microscope, it is seen to consist almost 

 entirely of a mass of minute lath-shaped felspars and microliths, 

 with little or no interstitial material, while porphyritic crystals of 

 any kind are practically absent. 



Augite probably existed as granules, and its presence may be 

 inferred from the small chloritic grains which occur between the 

 felspars. The felspars are twinned usually but once, although 

 occasionally they show albite-lamellation. The symmetrical ex- 

 tinctions range from 0° to 5° on either side of the twin-plane, 

 indicating oligloclase as the species to which the felspars belong, 

 and showing that these microliths correspond more or less exactly 

 with those occurring in the more coarsely- crystalline andesites 

 described above. This rock may be said to have a typical pilotaxitic 

 structure (PI. XXY, fig. 2). It is but feebly vesicular, for a very 

 few small vesicles occur, which are filled in the usual manner with 

 pale-green chlorite in fibrous aggregates. The fel spar-microliths show 

 an arrangement that roughly approximates to parallelism, indicating 

 in a measure the direction of flow, but many of the crystals are 

 curved or broken ; and from this, taken in conjunction with the 

 comparative irregularity of their arrangement, it would appear that 

 the rock must have been in a fairly-viscous condition immediately 

 prior to its consolidation. Only a very few crystals of a porphyritic 

 character occur, and the biggest, an oligoclase-andesine felspar, 

 measured -7 by -5 mm. The felspars were partly decomposed, with 

 the formation of epidote. 



A specimen taken from the surface of a flow in the lower 

 andesites, at a point a little below the rhyolites, proved to be a 

 fluxion-breccia. The rock is made up entirely of andesitic 

 material, mostly pumiceous, with a flow-structure developed between 

 the fragments. These fragments are identical, in every respect, 

 with those occurring in the pumiceous tuffs described on p. 241. 



(ii) The Fragmental Rocks. — The fragmental rocks of ande- 

 sitic character are largely in excess of the flows, and usually are 

 fine-grained, well-banded rocks of a buff to yellowish-green colour. 

 Many samples were collected from the lower and upper andesites ; 

 under the microscope they were seen to consist of alternating 

 coarser and finer bands of sedimentary material, and to be composed 

 of angular grains and fragments of quartz and felspars. The larger 

 fragments are set in a finer matrix of felspar-microliths and broken 

 felspar-crystals. 



