PETROGRAPHICAL NOTES. 8i 



felspars that often attain a length of two millimetres. Veins and 

 patches of pink calcite are also conspicuous. Here again, in the mi- 

 croscopic section, the broken condition of the felspars is such as to 

 cause doubt as to whether this rock was originally of the nature of a 

 tuff or a lava. In this case, however, it often appears that the felspar 

 crystals have been broken since the original consolidation of the rock, 

 for the fragments lying side by side ran frequently be referred to one 

 crystal. One crystal, originally a continuous prism two millimetres 

 long, is now broken into as many as sixteen fragments. At other 

 times the crystals have been shivered into a confuse mosaic, This is 

 the only unaltered mineral that remains in this rock, of which, however, 

 it makes up by far the largest proportion. The rock differs from all 

 the other specimens in containing neither hornblende nor leucoxene, 

 nor epidote. The spaces between the felspar fragments are occupied 

 by nothing but chlorite, calcite, and some small grains of iron ore, pro- 

 bably all of secondary origin. The rock must have been originally 

 less basic than the usual type. The specific gravity is decidedly 

 below the average, being only 283. Whatever may be the origin of 

 this calcite and chlorite, they have by their yielding nature protected 

 from the effects of dynamo metamorphism the felspar fragments and 

 aggregates which they isolate from one another. This mineral shows 

 the broad twin lamellae of labradorite, and although broken, it is fresh 

 and unaltered. 



Noith of this rock we get more of the darkchloriteschists, then a 

 paler variety D with large well-shaped crystal of pyrites (speci- 

 men yVV). Next comes the peculiar green jasper C, described on 

 page 68, possibly also of tuffaceous nature. Lastly we find a bed 

 of fine-graiued compact green diabase (specimen r'ii) resting 

 upon the slates A. In the field this rock .has a jointed appearance 

 and weathers into spheroidal masses very much after the manner 

 of a basalt. It has a specific gravity of 3*04. Under the micros- 

 cope this rock exhibits one of the most extreme types of altera- 

 tion. A few fragments of the prismatic felspars are sufficiently well 

 preserved to show by their small dimensions that the original rock 

 G ( 81 ) 



