on the prmcipal Ge^sirs of Iceland. 395 



metres to the east of the Geysir, and into which a part of the 

 water that overflows after each eruption run, present a fact of 

 the same kind: in the midst of a network of silicified plants 

 forming several layers, the total thickness of which exceeds 

 three metres, we ibund small portions of silex, which pos- 

 sessed all the properties of precious opal, as long as they are 

 strongly hydrated, but which only preserve iheir vivid colours 

 while they remain immersed in water, or are kept from dryinfr. 

 May we not hence conclude that the opals and chalcedonies 

 of some ancient volcanic districts owe their origin to pha?no- 

 mena similar to those of the present geysirs of Iceland ? 



The left side of the small ravine spoken of above, which 

 bounds the deposits of the ancient extinct geysirs, did not 

 present chalcedonous wood, but we extracted from it birch 

 stems transformed into very friable white silica, which ap- 

 peared to have been derived from individuals larger than 

 those we meet with at the present day in Iceland. Some spe- 

 cimens of these woods, in which the cells ahd vessels may still 

 be observed under the microscope, are penetrated in various 

 directions with small loots which appear to belong to the 

 same species ; we might without doubt thence conclude that the 

 large stems were already in a state of advanced decomposition 

 when the silica came and penetrated into their tissue,— a cir- 

 cumstance which would perhaps explain the reason why these 

 woods have not been converted into chalcedony, but into pul- 

 verulent silica. 



It is at ail events certain that a birch-wood once existed 

 between the ancient and recent geysirs ; and, as these trees 

 cannot live where the soil is covered with siliceous concre- 

 tions, it must l)e admitted that these two classes of fountains 

 did not exist simultaneously, and that the recent geysirs suc- 

 ceeded the primitive geysirs, when the orifices of these last 

 became insufficient to give a passage to the masses of water 

 which had to be cast forth*. 



With respect to the duration of the ancient phsenomena, 

 the depth of twenty-five metres at least of the soil which they 

 have formed, shows that it must have been very considerable. 

 In fact, the concretions deposited by the present geysirs have 

 not yet exceeded a tliickness of four to five metres; and all 

 seems to prove that, within the historical period of Iceland, 

 the increase of this thickness, which is distributed over an ex- 

 tended surface, has been very small, so that we must attribute 

 a \ery high antiquity to the recent geysirs themselvesf. 



♦ A very detailed descri|Uion of the districts formed by the ancient and 

 recent geysirs has been given by M. liobcrt in the geological part of the 

 Voya;;e of the Northern Scientific Commission to Iceland. 



t As it is almost impossible, in order to prove the thickness of deposits 



