178 ON THE CONCRETIONARY AM* Of the spheroidal Structure. 'Die 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 serve, 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-