202 REV. J. MILNE CURE AN. 



rock under examination there is abundant evidence that some of 

 the crystals have had two separate growths, but whether this 

 took place after consolidation is not so easy to decide. On Slice 

 11 there is a dome-shaped zoned crystal, that shows a bright 

 border when the centre is extinguished. As the line of extinction 

 for the central parts is passed, and the stage rotated, the zone of 

 extinction is seen to travel outwards without any kind of break 

 until it reaches the edges of the crystal. It would seem that this 

 particular crystal grew very slowly, from a magma that was itself 

 slowly altering through the removal of successive crops of crystals. 



Slice 12. — Fine-grained Basalt, near the Race Course, Orange. 

 This is a portion of a basaltic stream that flowed down from Mt. 

 Canoblas, about six miles distant. In hand specimens the rock 

 is of a rather bright blue-black colour. It breaks with a con- 

 choidal fracture and the clean fractured surfaces show a finely 

 crystalline glistening structure. None of the constituent minerals 

 are visible to the unaided eye. When large masses of this rock 

 are broken, it is interesting to notice how large drops of water 

 are set free, spreading on the surrounding stones like the large 

 drops of rain that precede a thunderstorm. This is noticeable 

 where the stone shows no trace of decomposition, at a depth of 

 twenty feet from the surface. Under the microscope in ordinary 

 light, the rock shows itself as a felted mass of plagioclase crystals. 

 Some of these crystals are cruciform in shape, and sometimes three 

 or four individual crystals aggregate diverging from a certain 

 point. These crystals seem equally distributed through the slide, 

 and show no disposition to form a Auction structure. The felspars 

 appear to be of two generations, the large felspars of which there 

 is only one good example on the slide being allogenic and much 

 corroded. In polarized light allotriomorphic granules of the usual 

 ferro-magnesian minerals, augite and olivine, are recognized, 

 wedged in between the felspars. There is a fair quantity of 

 interstitial glass, mostly showing incipient devitrification. Mag- 

 netite occurs in three forms — first in perfect cubes and grains 

 penetrating by turns every mineral in the slice, secondly, as micro- 



