72 Mr. Caldcleugh on the Geology of Rio de Janeiro. 



tained from such a situatioHj the specimens of siliceous stalactite which are 

 now sent to the Society. 



At the point of junction of the two beds of gneiss these siliceous stalactites 

 are in great abundance. 



The absence of hot springs makes the occurrence of these stalactites of very 

 considerable interest; and I cannot avoid offering an hypothesis to explain their 

 formation. In Brazil there is constantly a trickling of water down the bare 

 sides of the hills^ which from evident causes acquires a very considerable in- 

 crease of temperature. From having crawled through these little rills on my 

 hands and knees, I can give a pretty correct idea of their temperature, which 

 often reaches 140° or 150° of Fahrenheit. This warm water descending on 

 decomposing strata of gneiss, such as is the case with that from which these 

 specimens are taken, seizes the potash of the felspar, and then acts upon the 

 quartz ; and the water evaporating, deposits the silex in the form of a stalactite. 

 Some of the hot springs or geysers of Iceland, which form the stalactite, do 

 not reach the point of boiling; and perhaps the quantity of silex dissolved, (the 

 inverse of what is shown to be the case with carbonate of lime,) may in a great 

 measure depend on the temperature of the alkaline solvent. I do not know 

 whether the presence of potash in these specimens confirms this conjecture. 

 In some specimens the points are perfectly opaque, but the base puts on a 

 hornstone appearance. 



With the exception of the specimens from warm springs, recent siliceous 

 formations are comparatively scarce, but by no means unknown. If, however, 

 the more frequent occurrence of siliceous stalactite is to be looked for, it may 

 probably be in warm countries, where the agents of destruction and reproduc- 

 tion are more active than in the colder climates of Europe. 



