514 EGG. GEOLOGY. 



tions where this series is found. The concretions by 

 which it is characterized are seen either protruding from 

 the mass of the cliff, or detached by the wearing of the 

 surrounding parts and lying on the beach, where they 

 remain with little sensible change. They are either irre- 

 gularly spheroidal, or bounded by several portions of 

 smaller spheroids, and consist, like the surrounding 

 sandstone, of grains of quartz, but more strongly aggre- 

 gated ; having in some cases, when broken, the aspect of 

 the more compact sorts of quartz rock. Although com- 

 monly smooth on the surface, they are occasionally 

 cracked into irregular polygonal forms resembling those 

 of the septaria, the cracks being wide at the surface but 

 gradually diminishing at the bottom, and seldom penetra- 

 ting to more than an inch in depth. These balls have 

 been supposed to arise from crystallization, and to be 

 in themselves a sufficient proof that the strata which 

 contain them have been produced by that process. On 

 this I shall only remark that there is a wide interval 

 between the crystallized and the concretionary structure, 

 whatever analogy may exist between them ; and that the 

 present is not the only variety of this mode of aggregation 

 still unexplained, and still demanding the sagacity of 

 geologists. 



The trap rocks which are interposed among those now 

 described, present also several varieties both of general 

 aspect and of composition. Considerable ranges of columns 

 are to be seen in many places, being abundant toward the 

 east side of the island, and alternating with other forms 

 of trap as well as with the above mentioned strata. 

 These, as far as I examined them, are composed of dark 

 blueish and black basalt, and sometimes contain nodules 

 of mesotype; an occurrence not very common in basalt 

 or in any of the more compact traps. They are often 

 jointed, and very regular in their forms. At the extreme 

 point of the island a considerable range, of forty or fifty 

 feet in height, occurs at the level of the sea, the base of 



