CRYSTALLINE STRUCTURES OF ROCKS. 179 



roids of large dimensions are found attached in pairs 

 by a cylindrical stein, and imbedded in the surround- 

 ing rock ; from which they are easily separated after 

 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, and in other limestones, no explanation 

 of their origin has been suggested. These also occur 

 in the vicinity of trap rocks, in my own experience at 

 least. 



The smaller kinds of spheroidal structure are more 

 numerous, and present greater variety. In the sili- 

 ceous schist of the Shiant isles and Scalpa, it is as- 

 certained by decomposition, that the internal structure 

 consists of small aggregated spheroids ; the intervals 

 of which, being of a different nature, become con- 

 verted into clay on exposure, leaving a botryoidal sur- 

 face. In the fresh rock, this cannot be suspected. 

 The softer shales of the former islands are also fre- 

 quently found to consist of an aggregation of sphe- 

 rules 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, they assume, in some places, an imperfect 

 spheroidal tendency ; which gradually becomes more 

 perfect where they approximate to the trap; while 

 their substance, at the same time, is converted into 

 an anomalous stone resembling those cherts which 

 have been sometimes called hornstone. An inequality 

 of the internal texture is here also ascertained, by 

 the botryoidal surface which these assume on exposure 

 to the sea. 



N 2 



