Park. — Notes on a Quartz Mica-diorite. 



341 



In one example the composition plane was inclined at an 

 •angle of 18 degrees to the orthopinacoid cleavage-cracks. A 

 few plates show a fibrous structure, apparently due to aggre- 

 gation of needles with approximately the same orientation. 

 Between crossed nicols the needles do not extinguish at the 

 same moment, and the result is a display of banded colours 

 when the stage is rotated. The hornblendes are compara- 

 tively free from enclosures, but in the twin phenocryst 

 mentioned above there is enclosed a well-developed plate of 

 feldspar. 



Biotite has been very abundant in the rock when fresh. 

 It is now mostly altered to chlorite, being represented only by 

 dark-brown irregular cloudy patches with frayed edges. The 

 alteration shows initial encroachment along the cleavage- 

 cracks. Enclosures of magnetite and quartz are common. The 

 ■chlorite contains epidote, augite, and apatite as alteration 

 products of biotite. 



The quartzes occur in crystalline grains and interstitial, 

 with coloured enclosures. The former may be of primary 

 origin ; the latter is undoubtedly of secondary generation. 

 Fluid lacunae are common, polarisation colours brilliant. 



A chemical analysis of this rock by Mr. W. H. Baker, 

 B.Sc, of the Thames School of Mines, gave the following 

 results : — 



SiO a 



ALA 



FeO 



Fe 2 3 



CaO 



MgO 



K.,0 



NaoO 



H,6 



60-21 

 12-28 



13-84 



6-72 

 1-96 

 1-32 

 2-55 

 1-16 



100-04 



The percentage of iron-oxides is abnormally high, and is 

 ■due to the abundance of ferro-magnesian silicates. The silica 

 is somewhat below the normal. 



Locally the rock is known as granite, to which it bears a 

 strong resemblance. It is a handsome and durable building- 

 stone, very tough and hard, perhaps too hard for hand-dress- 

 ing where the rate of wages is high, but with mechanical 

 dressing-machines it should be produced at a rate to compete 

 successfully with imported stone. 



