394- M, Descloizeaux's P/iijsical and Geological Observaiwns 



disintegrated on the lower part, where it presents numerous 

 traces of having been acled upon and undergone a slight altera- 

 tion by the thermal waters, but it is tenacious and tolerably 

 homogeneous towards the summit. The height above the 

 plain of this hill, which has the name of LaugafjalJ, may be 

 estimated at seventy-five metres. From the foot of its eastern 

 escarpment, an undulating district extends for a length of 

 about 350 metres, entirel}' composed of ancient siliceous con- 

 cretions, slightly altered and friable. This district, the highest 

 hills of which rise to nearly twenty-five to thirty metres' 

 elevation above the plain, is pierced with a multitude of holes, 

 from which steam and the vapour of sulphuretted hydrogen 

 still escape; small quantities of alum and sulphur are deposited 

 on the edges of some of these holes, <me of which is situated 

 at the very foot of the trachytic hill. 



The existence of an ancient siliceous district, so extensive 

 as that which 1 have just mentioned, certainly appears attri- 

 butable to numerous extinct geysirs, the abundant deposits 

 of which have at length completely stopped the orifices from 

 which their eruptions took place. The lower part of this 

 ancient district is separated from the deposits of the present 

 Great Geysir by a small ravine of two metres in depth, situ- 

 ated at ten metres from the basin of the geysir, and directed 

 nearly from south to north. The sides of this ravine, from 

 whence acid vapours are also disengaged at several points, 

 discloses both in the ancient and modern formation, numerous 

 layers of siliceous concretions of a very varied aspect, distri- 

 buted in the midst of a bolary clay in which the red colour 

 predominates. 



On the right side which bounds the deposits of the present 

 geysir, and in a very plastic clay, we meet with small stems of 

 birch-trees, of the size of those which still grow in the country, 

 entirely silicified and converted into chalcedony ; the clay itself 

 is full of small crystals of white pyrites, formed from the iron 

 which it contains and of the sulphur of the vapour springs. 

 Above the chalcedonous stems, and ayiproaching the most 

 recent strata of concretions, we now only find birch-leaves 

 and a network of herbaceous plants, forming a sort of siliceous 

 travertine. In this same part, and always in the middle of a 

 reddish claj', I have observed a thin, very regular, and exten- 

 sive layer of zoned chalcedony; this substance, which is trans- 

 hicid as long as it remains immersed in the moist clay, becomes 

 opake and of an enamel white by a simple desiccation in the 

 open air, without being able to re-acquire its transparency, 

 even on a prolonged immersion. 



The banks of the little river the Beina, which flows at 300 



