Prismatic Structure in Sandstone, fyc. 255 



to have no particular connexion with the question now under 

 consideration. 



It is proper to remark, however, that this bed of ironstone, 

 as it is called, might equally be termed a shale, in which the 

 red oxide of iron abounds in an unusual degree. Such a com- 

 position of clay and iron is, indeed, that of all iron stones. In 

 this place many of the shales are highly ferriferous, being of 

 red, yellow, and purple colours ; so that while the exact limits 

 of ferriferous shale and argillaceous ironstone cannot be defined 

 in the abstract, neither, in this place, can it be said that many 

 of these beds are not formed of the latter substance. The 

 shales also pass so gradually, in respect to composition, into 

 the columnar ironstone which occurs near them, that the 

 differences are often not perceptible. The columnar bed under 

 review might, indeed, with equal propriety, be called a ferri- 

 ferous shale ; or else the shales of the same character should 

 rank, like it, with the ironstones. 



I must remark that the surface of the columnar ironstone 

 in this place being exposed, there is ready access, not only to 

 this bed itself, but to all the surrounding rocks. These rocks 

 are sandstones, shales, and limestones, of various characters ; 

 all of them appertaining to that series of red sandstone which 

 forms this quarter of Arran, and which extends across the 

 whole of Scotland to the eastern shore. Besides these, veins 

 of trap occur in the immediate neighbourhood ; some of them 

 intersecting the strata, and others following the same parallel 

 course, so as to appear interstratified with them. It ought still 

 to be observed, that although masses of superincumbent trap 

 are not found at this particular place, they abound in other 

 places in Arran, not far off; forming, indeed, a considerable 

 portion of this island. As the reasoning which it is intended 

 to apply to this phenomenon applies equally to the case next 

 to be described, I shall not now enter on it, but defer the whole 

 argument till the history of this also has been given. 



In the island of Rum there is a mass of sandstone, similar 

 to that of Sky, which, in some places, is covered by trap rocks 

 of various characters. Under the hill of Scuirmore, this sand- 

 stone lies under that basaltic rock so remarkable for the quan- 

 tity of heliotrope which it contains. In one place the bed is 



