430 SCALPA. GEOLOGY, 



after forming an accurate series of specimens ; the more 

 so, as no idea of them can be communicated by words, 

 and as it would be impossible to give sections, without 

 adopting a magnitude of scale and a multiplicity of en- 

 gravings inadmissible in a work of this nature. 



The sandstone is sometimes red, while at others it 

 passes into grey and into lead blue. Occasionally it 

 approaches to the schistose structure, the laminae being 

 determined by interposed scales of mica. The same schist 

 which accompanies the beds formerly described is also 

 found here, and in the same irregular manner; but it 

 appears chiefly to prevail in the lowest part of the mass. 



With these and with grey quartzy sandstone, there 

 occur beds of a blackish hard rock of the nature of 

 siliceous schist; usually distinguished by laminse alter* 

 nately lighter and darker, and prevailing chiefly in the 

 vicinity of the limestone* Among them are also to be 

 seen portions of that schist which is characterized by the 

 globular structure, and of which examples will hereafter 

 be described in the Shiant isles. As the mode of 

 weathering in this place explains the origin of the pro- 

 jecting globules, it may be briefly described ; particularly, 

 as in the instance just alluded to, the same explanatory 

 circumstances do not occur* 



In a recently broken specimen there is a perfect uni- 

 formity of texture without the least indication of the 

 globular conformation. As the rock begins to weather, 

 it discovers to the depth of an inch or more, grey globular 

 masses imbedded in an ochrey-coloured basis ; the two 

 being at first scarcely distinguishable in hardness. By 

 degrees the yellow parts soften and at length are washed 

 away by the sea, while the globular and harder grey 

 parts continue; destined ultimately to project in the botry- 

 oidal groups described more fully in speaking of the 

 Shiant isles. It will hereafter appear that the argilla- 

 ceous porphyry of Arran which undergoes the same 

 chants, is constituted in a similar manner and owes its 



