94 THE WONDERS OF GEOLOGY. Lect. I. 



in a pure or combined state, nearly one-half of the solid 

 crust of the globe. The flints from our chalk cliffs, the 

 boulders and gravel on our sea- shores, and the pebbles of 

 agate, quartz, and chalcedony, are well-known examples of 

 the usual varieties of silex. 



I scarcely need observe, that this nodule of flint, obtained 

 from a neighbouring chalk-pit, has once been in a soft or fluid 

 state; for there are many sharp impressions of shells, and of 

 spines of an echinus, on its surface.* We have already seen 

 that water impregnated with carbonic acid gas is capable 

 of holding lime in solution, and that travertine, limestone, 

 and other calcareous deposits, have originated from this 

 agency ; and although, even in the present advanced state 

 of chemical knowledge, we are but imperfectly acquainted 

 with the process by which any considerable proportion of flint 

 can be dissolved in water, yet we have unquestionable proofs, 

 that the solution of siliceous earth has been effected by 

 natural processes, on a very extensive scale. At the 

 present moment, Nature, in her secret laboratories, is still 

 carrying on a modification of the same process ; and of this 

 fact we have remarkable instances in the Geysers of Ice- 

 land, and in the Azores, and in New Zealand. A high 

 temperature appears necessary to enable water to hold in so- 

 lution a large quantity of silex ; for we find that the thermal 

 springs of volcanic regions are the only agents by which 

 siliceous deposits and incrustations are at present pro- 

 duced. The difference between the modern siliceous sinter. 

 and the flint of the cretaceous strata, appears to be refer- 

 able to the subaerial deposition of the former, and the sub- 

 marine formation of the latter. 



48. The Geysers of Iceland. — The Geysers, or in- 

 termittent boiling fountains, of Iceland, have long been 



* See " Thoughts on a Pebble; or, a First Lesson in Geology." 

 Seventh edition : 1846. 



