EXCURSION XI. 155 



We can easily pass from this point into the lower part 

 of Moneadh-Mhor glen. At the opening of this glen, in 

 the bed and banks of the stream, and of the lead of water 

 connected with the large mill adjoining, there are two great 

 beds of pitchstone. They are finely exposed, and exhibit 

 very strikingly those transitional appearances already alluded 

 to as marking the relations of this rock to hornstone and 

 claystone. 



Associated with the pitchstones there are claystones, horn- 

 stones, quartz rock, and porcellanite. Hornstone that is, 

 chalcedonic or jaspery chert seems to be the link between 

 pitchstone and claystone. Hornstone and pitchstone are both 

 almost entirely siliceous; the difference consisting in the 

 colour and degree of toughness arising from a slight change 

 in composition, or variation in the rate at which they cooled, 

 or from both. By this change pitchstone passes into horn- 

 stone. In this hornstone a great many light-coloured spots 

 with dark centres are gradually developed ; and bands of this 

 variety succeed the common hornstone. Next to this there 

 follows a hard quartzose claystone, and the series terminates 

 in the common claystone of open texture, like that of the 

 Corriegills shore, already described. The spots or specks are 

 minute spheres, most probably of felspar or quartz, or of 

 both, and have probably originated in the manner suggested 

 at the end of Art. 39. They present a close analogy with 

 the spherulitic claystone of Corriegills ; but the radiated 

 structure of the latter does not exist here. Even the larger 

 spherules of the pisolitic hornstones do not exhibit this 

 structure. The quartz-rock at the upper end of the higher 

 pitchstone vein is probably only an altered sandstone. The 

 porcellanite alluded to above is a white substance, varying 

 from a dull earthy aspect to that of a white enamel. It 

 occurs of considerable thickness along the outer surface of 

 the pitchstone, and is clearly due to a decomposition of this 

 rock. The incipient stages of this decomposition shew a 

 structure in the rock which otherwise we should not have 



