484 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
surrounding red sandstones. The paste of this rock is a granular tuff, formed 
evidently of comminuted diabase. 
“The blocks imbedded in it are angular, sub-angular, and rounded, of all 
sizes up to masses of a yard or more in diameter. 
“T observed among them pieces of diabase (these form the great majority), 
red sandstone, grey flagstone, gneiss, and abundant fragments of the augite, 
which I send you. 
“Some of the diabase blocks were of great size, but as veins of a similar 
rock traverse the neck, it was not always possible to tell which were parts of 
veins and which were really detached blocks. The augite fragments are all 
rounded, as if they had undergone .considerable trituration before they came 
to rest. Here and there they show a rough cleavage-face, which may have 
been produced by their striking against another ejected block. They 
vary in size from mere seed-like grains up to blocks at least 8 or 9 inches 
in diameter. 
“No sign of fusion, or even of good baking, was to be seen in the bits of 
imbedded sandstone in the agglomerate. I should add that I did not observe 
any minerals associated with the augite, except such as had resulted from the 
alteration of the tuff. 
“There cannot be the least doubt that the fragments of augite are true 
ejected blocks, and did not originate in the matrix where we found them, 
“They are by no means the only examples to be met with among the 
Scottish tuffs, though they are very much larger than any I have before 
observed. Next to these in size, were some which I found in a neck on the 
shore to the south of Fairlie in Ayrshire, belonging to the Lower Carboniferous 
volcanoes.” 
These masses proved on examination to be an augitic glass, somewhat 
similar to, but neither so much vitrified or flawed as the Elie or Giant’s Cause- 
way specimens. 
It will be observed that GEIKkIE remarks on the granular pieces suggesting 
olivine to him; which mineral the Elie specimens were considered by the 
writer to be, until their analysis established the occurrence of augite in this 
peculiar allomorphic form, 
The resemblance to olivine is, in the John o’ Groat’s specimens, by no means 
—even in the most granular portions—so striking as in those already men- 
tioned ; while some portions of the specimens show, unchanged, the features of 
ordinary augite. 
This fact of the specimens presenting a changed and an unchanged portion 
—having been caught or arrested in the middle of the process of change— 
renders them certainly far more interesting, from the amount of information 
they convey, than those in which the glassy condition is perfect. 

