| 
| 
| 


PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 483 
no doubt it is augite. Some of the rough granular pieces suggested, at first, 
olivine to me. But I found the granular character to be exceptional, and the 
mineral is fusible, and with hardness equal to 5°5 
“Tt is by far the most interesting volcanic augite I am acquainted with.” 
On the 4th December he writes :— 
“The specimen I send you is one of a number of pieces of what seems to me 
to be a form of augite, which I have found in the most northerly volcanic rock 
on the mainland of Scotland. 
“This occurs on the shore, half a mile to the east of John o’ Groat’s 
House. 
“While the whole of that part of Caithness lies upon the so-called ‘ Caith- 
ness flagstones’ of the Old Red Sandstone, the coast at the locality in question 
affords an admirable section of the red sandstones, with blue and gray flags 
and shales (sometimes with well preserved ichthyolites) which form the 
highest visible portion of the Caithness flagstones. 
“No igneous rocks of any kind are interbedded among these strata, or in 
any other part of Caithness. 
“Tt was, therefore, with the utmost surprise that, when examining the coast- 
line in the year 1874, in company with my colleague in the Geological Survey, 
Mr R. N. Peacu, I came upon a well-marked volcanic ‘neck’ or pipe, which 
had been drilled through the John o’ Groat’s sandstones. 
“T was not at that time aware of any truly contemporaneous igneous rocks 
associated with any part of the Old Red Sandstones in this northern region, 
_ though I knew of the existence of what are called ‘trap dykes’ both in 
Caithness and among the other Orkney Islands. 
“But in prolonging my observations into the opposite island of Hoy, I saw 
abundant evidence of the intercalation of true lavas and tuff, at the base of the 
Upper Old Red Sandstones. 
“These sheets of volcanic material were found to lie in complete dis- 
cordance upon the Caithness flagstones. I found among them some material 
exactly similar to that which fills the John o’ Groat’s neck. I have no doubt, 
therefore, that this neck marks the chimney of one of these Upper Old Red 
Sandstone volcanoes. 
“T again visited Caithness last summer, accompanied by another Survey 
| colleague, Mr Joun Horne, and made further notes about this interesting 
locality. 
“The neck is irregularly oval in shape ; the diameter, on this edge trun- 
cated by the sandy beach, being about 300 feet. The sandstone round it is 
Somewhat hardened and jointed, as may usually be observed in similar cases, 
The whole space of the neck is filled up with a coarse tumultuous agglomerate 
ib a dirty green colour, which makes it stand out in strong contrast to the 
| 
