Dr. Mac Cu l l g c h on the Hill of Kinnoul. 225 



surrounded by the solid rock on all sides. It is more common for 

 the boundary of the quartz in the immediate vicinity of the trap to 

 be formed of various zones of coloured chalcedony. The quartz 

 in this case assumes a peculiar well-known aspect, and is called in 

 the Wernerian nomenclature, amethyst, although most commonly 

 of a white or watery appearance. Different zones of chalcedony 

 and quartz will even at times succeed each other in the same nodule. 

 But the imbedded mineral from which this place has acquired 

 its greatest celebrity, is the agate, or coloured chalcedony, with 

 which it abounds, but which it possesses only in common with many 

 other places in Scotland. The nodules of this substance vary ex- 

 ceedingly both in size and colour, and their general aspect is much 

 too well known to need any description ; yet a few circumstances 

 respecting them deserve to be considered, as they involve difficul- 

 ties which it is incumbent on any general theory of the formation 

 of these rocks to explain. Their external surfaces, I believe inva- 

 riably, bear those marks of indentation by the surrounding rock 

 which determines their posteriority of formation, or at least their 

 posteriority of induration, to that of the rock in which they are 

 imbedded. Their interna! structure is also most commonly zoned, 

 with irregularities corresponding to those of the external boundary ; 

 but in some cases they exhibit a complication of structure, which 

 as it cannot be well described in words, I have ventured to repre- 

 sent in the accompanying sketches. In the first example, a sta- 

 lactite may be observed occupying a portion of a hollow cavity, 

 marking as in the case of the larger quartz cavities described above, 

 the gradual deposition of siliceous matter by infiltration.* The 

 change of disposition in the zones in the figures N° 2 & 3, f seems 

 to be the result of a similar process, the horizontal parallel lines 

 * Pi. io. N° l + Pi. io. N° 2 & 3. 



