95 



VII. — Vesicular Basalt. 



Rock 4. 

 Leg-of -Mutton Lake. 



Although this specimen was not found in situ, yet it 

 represents the usual type of rock found in the vicinity of this 

 lake, and to a smaller extent in the Valley Lake. 



Macroscopic Characters. 



In hand specimens the rock is steel-grey in colour, con- 

 taining many vesicules of variable dimensions. Some of the 

 vesicules contain small crystals of aragonite, others a thin 

 layer of calcium carbonate or lime, with a small amount of 

 foreign matter included. A few small visible phenocrysts of 

 olivine are seen scattered through the rock. Tachylytic struc- 

 ture is not developed to the same degree in this rock as it 

 is in the others of the same group. 



Micro SCO pic Char a c t trs . 



Hypocrystalline porphyritic and medium-grained rock, 

 containing phenocrysts of olivine in a groundmass of plagio- 

 clase felspar, with granules of augite, magnetite, and a 

 little glass. 



This rock is particularly rich in microlite felspar of large 

 dimensions, which are fairly well preserved, giving symmet- 

 rical extinctions up to 32°, which suggests that it is a medium 

 labradorite. They are fairly well twinned, and in a great 

 many instances are arranged into radiating aggregates. Many 

 of the microlites are obscured by the numerous inclusions of 

 magnetite in the form of minute grains, and apatite. 



The granules of augite, which are brown in colour, are 

 in close association with the felspar laths, but they have un- 

 dergone a great deal of decomposition and are partially oblit- 

 erated with minute granules of magnetite, ilmenite, and some 

 glass, otherwise an intersertal structure would be developed. 



The glassy interstitial material is dark-brown in colour 

 and crowded with numerous granules of magnetite, which, in 

 part, renders it quite opaque. 



The only phenocryst of the rock is olivine, which has 

 undergone partial decomposition to serpentine, and is parti- 

 ally absorbed by the magma. 



The crystals are quite clear, but cracked about a great 

 deal. They contain fairly large grains of magnetite and 

 irregular inclusions of brown glass. 



Owing to the absence of phenocrysts of augite and the 

 predominance of moderately large plagioclase microlites, the 

 rock is not so fine-grained as the usual type of basalt, but 



