494 



THE WONDERS OF GEOLOGY. Lect. V. 



and that upon it is superimposed the Oxford clay ; next 

 follows the Middle Oolite; which is surmounted by another 

 thick argillaceous deposit, the Kimmeridge clay ; and upon 

 this is the Portland or Ujiper Oolite. 



7. Oolite, or roe-stone. — The limestones of this for- 

 mation have very generally a peculiar structure, being 

 composed of an aggregation of small rounded grains or 

 spherules, presenting some resemblance to clusters of 

 small eggs, or the roes of fishes ; whence the name oolite, or 

 egg, or roe-stone, which is now applied, not only to lime- 

 stones possessing this character, but also to the entire series 

 of deposits which intervenes between the Chalk and Weal- 

 den above, and the Lias below. On the Continent the group 

 is generally termed " Terrains Jurassiques" from the 

 Jura mountains that divide France from Switzerland being 

 largely composed of these deposits. 



The oolitic structure is not however confined to this 

 division of the secondary rocks ; for it occurs in tertiary, 

 and also in some of the most ancient sedimentary strata. 

 It consists of an aggregation of grains or globules of calca- 

 reous matter, composed of concentric laminse which com- 

 monly have an atom of sand, or a minute shell or coral, as 

 a nucleus. These globules owe their formation to the 

 deposition of successive spheroidal concretions around the 

 included body while subjected to the action of water in 

 which a rotatory motion is induced ; and the spheroids 

 continue to increase, till they become too heavy for further 

 transport, and then subside, and are consolidated by sub- 

 sequent infiltration. When the individual spheres are of a 

 large size, the aggregated mass is called pisolite, or pea- 

 stone. The springs near Carlsbad deposit a beautiful 

 conglomerate of this kind, some masses of which are suffi- 

 ciently compact to admit of being manufactured into boxes. 

 and other ornaments. Polished slices exhibit every variety 

 of sections of the concentric layers of which the concretions 



