70 Dr. Mac Culloch on the Concretionary and 



distinguished by a greater hardness of texture than the surround- 

 ing rock, whence they are easily separated as it wastes away. 

 Their own texture is also unequal ; and it not unfrequently happens 

 that the superficies is cracked into polygons. How far the influence 

 of trap may have tended to the production of these must be con- 

 jectured from the circumstances respecting the prismatic structures 

 of sandstone, from the fact that these spheroidal sandstones also 

 occur in the vicinity of trap. I may here add, that concretions of 

 large size have lately been brought from the new-discovered land 

 of South Shetland, consisting of the halves of very flattened sphe- 

 roids ; as if such figures had been cut through, according to their 

 equatorial diameters, by a sharp tool. 



In the argillaceous limestone, as well as in the accompanying 

 sandstones of Sky, highly flattened spheroids of large dimensions 

 are found attached in pairs by a cylindrical stem, and imbedded 

 in the surrounding rock ; from which they are easily separated on 

 its destruction, on account of their superior hardness. They bear 

 no resemblance to organic forms; and although they have also 

 been observed in other parts of Europe, no explanation of their 

 origin has been suggested. It need only further be remarked, that 

 these also occur in the vicinity of trap rocks. 



The smaller kinds of spheroidal structure are more numerous, 

 and present greater variety. In the siliceous schist of the Shiant 

 Isles and Scalpa, it is ascertained by decomposition, that the 

 internal structure consists of small aggregated spheroids ; the 

 intervals of which, being of a different nature, become converted 

 into clay on exposure, leaving a botryoidal surface. In the fresh 

 rock this cannot be suspected. The softer shales of the former 

 islands are also frequently found to consist of an aggregation of 

 spherules not larger than mustard-seed. In these cases also trap 

 is present ; and it is easily proved that the rocks in question were 

 once the ordinary shales of the coal strata, which, in undergoing 

 induration, have also experienced this change of structure. Where 

 some of the claystones of Arran are invaded by trap veins, it is 

 found that they assume in some places an imperfect spheroidal ten- 

 dency ; which gradually becomes more perfect where they approxi- 



