178 ON THE CONCRETIONARY AND 



Of the spheroidal Structure. 



The spheroidal structure is found under different 

 modifications; some of which are among the most 

 inexplicable phenomena of this nature which geology 

 presents. The explanation of those which approach 

 in their nature to crystallization, is not so difficult ; 

 and these examples servfc, in some measure, to con- 

 nect two processes, otherwise very different in their 

 natures. The large spheroidal structure of granite, 

 already mentioned, cannot with propriety be ranked 

 with this ; nor that which occurs in Trap, in Rum, 

 and elsewhere. 



In the secondary sandstones of Egg and other 

 places, there are found large spheroids imbedded in 

 the ordinary strata. These are distinguished by a 

 greater hardness of texture than the surrounding 

 rock, whence they are easily separated as it wastes 

 away. Their own texture is also unequal between 

 the centre and circumference ; 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 conjectured from 

 the circumstances respecting the prismatic structures 

 of sandstone formerly stated, and 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 spheroids ; 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 ac- 

 companying sandstones of Sky, highly flattened sphe- 



