NATURAL HISTORY SOCIETY OF GLASGOW. 237 



matrix that they can now readily be separated from the clay by 

 washing. 



Similar calcareous nodules, enclosing fossils, are not uncommon 

 in the limestone strata of our Scottish coalfields, but I know of no 

 strata containing them which has been acted upon in the same 

 peculiar manner as that observed at Glencart. It might have been 

 supposed that, as the shells of the mollusca in the Glencart nodules 

 were originally calcareous, they would also have been acted upon 

 by the carbonic acid which has removed the lime, but such is not 

 the case, as the shells retain their form and all their original fine 

 markings. The reason why they have been so wonderfully 

 preserved, under such, apparently, unfavourable conditions, is 

 to be found in the fact that the lime which originally entered 

 into their composition has been replaced by other mineral 

 elements, such as silica and alumina in part, with probably a 

 little sulphate of barytes, and which have resisted the action 

 of the carbonic acid. There is every reason to suppose that 

 this mineral change was effected on the shells prior to the strata 

 being acted upon by the waters containing the dissolving in- 

 gredients. The causes which led to these changes in the mineral 

 composition of the hard parts of the organisms may be both difficult 

 to trace and to clearly explain ; but it may, perhaps, have been due 

 to heat derived through igneous action, which, we know, was con- 

 temporaneous in this district of country with much of the strata 

 belonging both to the lower coal and ironstone series, and those 

 of the upper limestone group. The Geological Survey officers, in 

 the Memoir explanatory of sheet 22 of the Ayrshire coalfield, 

 notice both beds of trap tuff and sheets of basalt, which alternate 

 with these two divisions of strata in the Dairy district. 



In the section at Glencart there is found overlying the bed con- 

 taining the nodular cavities, a thin band of a grey siliceous deposit 

 wdiich has a flinty or horny texture. This stratum, which appears 

 to be a true hornstone, has been, I "am inclined to think, 

 deposited chemically by heated waters, which held the silica 

 in solution, and which had their origin in a spring, probably 

 issuing from some of the fissures connected with the volcanic 

 vents then active in the neighbourhood. The changes now 

 observable in the mineral composition of the fossils were, in 

 all likelihood, effected during this period, and it is also probable, 

 that at some after period, springs charged with carbonic acid, 



