127 



guishable under these circumstances. Arran, 

 and many other places in the Western isles, pre- 

 sent examples of this occurrence, in the siliceous 

 schists. 



In the cases to which the preceding remarks 

 apply, the general structure of the rock is con-, 

 cretionary throughout. The concretions are also 

 of limited size, varying from that of poppy seed 

 to the diameter of an inch. 



In other instances, the concretionary structure 

 is confined to particular parts of a rock ; or insu- 

 lated concretions are imbedded in an uniform mass. 



Such concretions are commonly of large size, 

 and generally in the form of obate spheroids. In 

 some rare instances, they are attached in pairs by 

 a cylindrical stem ; as in the limestone and sand- 

 stone of Rasay and Sky. 



In. others, they are cracked on the surface into 

 polygonal forms, which are, consequently, frusta 

 of pyramids. Of this, the sandstone of Egg pre- 

 sents examples. 



Where the spheroidal form results from the 

 desquamation of a prism, it is noticed under the 

 following head. 



