ARRAN. GEOLOGY. OVERLYING ROCKS. 395 



in assigning the same name to it. It may indeed be 

 called indurated claystone, but it may also with as little 

 error be considered a variety of clinkstone or of compact 

 felspar,* since it is not far removed from these substances. 

 As the term indurated claystone is however in many cases 

 very convenient, I shall for the present adopt it. 



As this rock continues to increase in hardness, it is 

 frequently characterized by an imperfect granular fracture 

 and more sandy texture, as well as by an occasionally 

 carious or cavernous aspect ; while it approaches so much 

 more nearly to clinkstone that the term arenaceous clink- 

 stone may, for want of a better, be used to distinguish it. 

 This term is expressive of its appearance, since it often 

 resembles sandstone so strongly as to have been mistaken 

 for it by mineralogists who have visited and described 

 Arran. There is however reason to suspect that this 

 appearance is the result of an incipient decomposition. 



In a further state of induration, and still retaining a 

 simplicity of aspect, it puts on other characters, becoming 

 so various in texture, colour, and general appearance, 

 as to be often a source of error to mineralogists, who 

 are easily led to confound the dark-blue, or speckled 

 varieties, with basalt or with fine greenstones. The pre- 

 dominant colours are pale ochrey white, fawn colour, brown, 

 and lead blue ; and the same colours occur in the ana- 

 logous rocks of whatever degree of induration. The latter 

 varieties are often remarkable for the deep stain of yellow 

 which penetrates from the surface ; the effect of the car- 

 bonization of the black oxide of iron whence the substance 

 derives its dark colour. With attention, such dark varie- 

 ties are generally to be distinguished from greenstones or 

 basalts ; since both the latter derive their colour from 



* I have here used the term compact felspar in common with other 

 mineralogists, although not convinced of its propriety; since the sub- 

 stance in. question appears to contain soda, while felspar contains potash 

 only. 



