

SKY. GEOLOGY. SECONDARY STRATA. 341 



entirely to disappear on the opposite one after a very 

 short indication of their existence. It is for the same 

 reason that they appear no more, either on the pro- 

 montory of Vaternish, or westward of that tract; their 

 dip having carried them far below the sea, so that 

 the trap alone occupies the whole visible land. The 

 accompanying section will illustrate the notion here 

 presented.* 



Having thus sketched the general topography and 

 position of these strata, as far as their obscurity admits, 

 it is necessary to examine the order of their succession, 

 and to trace them, by such indications as they afford, 

 from the limestone strata of Sligachan formerly described. 



Between these two places however there is such a geogra- 

 phical chasm, that the first step of this structure can only 

 be laid by analogy ; and in having recourse to Rasay for 

 this purpose, I need not revert to the apparent con- 

 nexions between this island and the opposite shore 

 so lately pointed out. It has been seen that the limestone 

 of Rasay, which corresponds to that of Sligachan, is 

 followed in immediate succession first by beds of a 

 micaceous shale containing numerous petrifactions, and 

 then by calcareous sandstone remarkable for its spheroidal 

 concretions, and of which the only conspicuous organic 

 remains are a belemnite and a large ammonite. This 

 sandstone forms a mass of great thickness, reaching to 

 the surface of the island and apparently containing some 

 portions of limestone, although the interference of the 

 incumbent porphyry materially impedes accurate exa- 

 mination. 



It must now be recollected, that the limestone of 

 Sligachan is separated from the rocks that follow, by 

 the interruption which that inlet causes. On its northern 

 shore, there appear two thin beds of sandstone, the 

 first white, and the second of a black colour, following 



* PI. XIV. fie;. 4. 



