164 REPORT — 1844. 



solved and afterwards deposited again by way of crystallization ; and the ap- 

 pearance of carbonaceous matter and of iron pyrites in the slate being, in 

 Scandinavia at least, always connected with the collection and crystallization 

 of carbonate of lime in large nodular masses, it appears that there must be 

 some causal connection between all these phagnomena. It is well known that 

 carbonic acid dissolved in water has the power of dissolving carbonate of 

 lime, and of depositing it again in a crystalline state whenever the carbonic 

 acid gas can escape ; and although geologists generally suppose the carbonic 

 acid to be derived from the interior of the earth, yet any free carbonic acid, 

 from whatever source it may originate, will have the same effect. I have 

 already shown before, that the first process in the spontaneous decomposition 

 of fucoidal plants of the present time, is the formation of a great deal of car- 

 bonic acid, and I therefore think it highly probable that the cai-bonic acid 

 which accompanied the decomposition of sea-weeds, has dissolved the lime of 

 the slightly marly clay, and collected it into large nodular masses. It appears 

 to me that both the detailed coincidence of the phaenomena observed at tlie 

 present time, with the facts observed in this large and important Silurian for- 

 mation, are a strong proof of the correctness of my views. 



As to newer formations of beds, where fucoidal plants have had a consider- 

 able influence on the chemical composition, I name with great hesitation the 

 lias slate of the coast of Yorkshire near Whitby. In fact I am not aware that 

 any impressions of fucoids have been found in this extensive formation ; but 

 then it is well known, that sea-weeds retain their form under very favourable 

 circumstances only, and that geologists generally pay very little attention to 

 those undefined plants which are considered to be of little use in determining 

 the age of the formation. Besides the want of fossil fucoids, the want of potash 

 in the lias slate of Whitby seems to be a serious objection to the influence of 

 sea-weeds on this formation ; but then, although the shale does not contain a 

 sufficient quantity of potash to make alum, yet it may contain a small quantity 

 of it, and I am not aware of any analysis of this shale. On the other hand, the 

 pyrites disseminated through the shale, the carbonaceous substances which it 

 contains, and the nodular concretions of carbonate of lime similar to those of 

 the alumslate in Scandinavia, offer no small points of analogy. We find, besides, 

 that the sulphuric acid in the ashes of the fucoids is frequently combined 

 with lime, as for instance in the Fuciis vesiculosus of our shores ; and the 

 spontaneous decomposition of this plant, when acting upon ferruginous clay, 

 would form a great deal of pyrites and a small quantity of potash, while the 

 lime would assist in forming the nodules of impure limestone. 



In the island of Bornholm the older greensand contains numerous beds of 

 coal, and in some beds an enormous number of Fuciis intricatus. The de- 

 posit contains no clay, and thus no potash could be retained*, but all the iron 

 of the formation is combined with sulphur in pyrites, which seems to be owing 

 to the same action. 



Lastly, a tertiary deposit contemporaneous with the subapennine forma- 

 tion, contains (all over the Danish peninsula) very large beds of alum earth. 

 This alum earth is black, contains much pyrites, and at the same time potash ; 

 the carbonate of lime in this formation is also collected in nodular masses ; it 

 is full of marine shells, but no fossil fucoids have yet been found in it. 



The Silurian alum slate seems particularly well-disposed to form gneissose 

 rocke by metamorphosis ; but before 1 show that this really has been the case 

 in the neighbourhood of Christiania, I must as a chemist beg leave to offer a 

 word upon metamorphosis in general. 



* By this expression the author refers to the insoluble combination of potash with clay de- 

 scribed in preceding paragraphs. — Ed. 



