ON CUPRIFEROUS TUFFS. 285 



MICROSCOPICAL CHARACTER. 



If macroscopic character and chemical composition alone were 

 taken as guides the true nature of the rock might remain hidden 

 in obscurity, but a whole flood of light is thrown upon its origin 

 by the microscope when applied to the examination of carefully 

 prepared thin slices of the tuff. After several unsuccessful 

 attempts to make thin sections of this rock by grinding down with 

 water and emery after previously boiling the specimens for days in 

 Canada balsam, Mr. C. Murton succeeded in preparing some good 

 sections of this rock by filing it down dry after the rock had been 

 hardened as before by boiling in balsam. 



The microscopic sections at once reveal its volcanic origin. 



The bulk of the rock is built up of rounded or subangular 

 particles of crystalline rocks chiefly if not wholly of volcanic 

 origin. Some are black and opaque but for minute lath-shaped 

 felspars, many of which are still so far free from decomposition as 

 to allow light to pass. Others shew various shades of green and 

 are more or less translucent, and others are of a brown pink 

 colour and some pale greenish-grey. The whole are cemented by 

 a grey, or more or less transparent base. Opaque intensely black 

 grains and crystals are sprinkled through the slide and are 

 observable in the volcanic particles as well as in the base. 



Slide No. la is seen to belong to a fragmental rock, the frag- 

 ments varying from the size of a small pin's head to that of 

 dust shot. The prevailing colour of the fragments is greenish- 

 grey and purplish-black. A fine crystalline pseudomorph of 

 copper is visible near the centre of the slide as shewn in the 

 drawing. The clear grain just above the copper pseudomorph is 

 quartz. The laige fragment of lava below the coj^per crystal 

 showing the lath-shaped felspar is of a reddish-black or reddish- 

 purple colour. The felspar is opaque from kaolinization. The 

 large fragment of lava to the right of the centre is a fragment of 

 purplish-grey and reddish-purple trachyte (?). The felspars are more 

 minutely crystalline than in the pi'evious species, and have four- 

 sided, or nearly circular shapes, enclosing dark spots, but there is no 

 tendency wliatever even at the edges to decompose into greenish 

 mineral. The elongated oval fragment of lava consists of micro- 

 crystalline lath-shaped felspar, with a great deal of pale greenish- 

 grey, and viridite-like decomposition material. Magnetic iron is 

 frequent in some quantity. To the left of the centre and a little 

 below it is a pale yellowish-grey fragment which to judge from the 

 cleavage may be a decomposed augite. There are besides in this 

 slide occasional fragments of a pale greenish-grey colour, of a 

 somewhat serpentinous appearance. This is made up of a great 

 number of little crystals of tabular shape, yellowish-grey colour, 

 and semi-translucent in a pale greenish-grey base shewing vanish- 

 ing outlines of lath-shaped felspar with occasional dark green 



