" Kjnahan — On Oldhamia. 343 



present is found, may be a mineral, and may even in part have 

 lost its original form ; but I contend that originally it was an 

 organic form, but now mineralized, or having on the original form 

 minerals built up. This subject was treated on nearly a quarter 

 of a century ago in a Paper by our member Mons. A. Grages, 

 who pointed out, in reference to the graptolites in some Ordovician 

 black shales, that they were not only mineralized, but that at 

 certain attractive places, such as points, bunches of crystals had 

 accumulated. It appears scarcely necessary to point out that in 

 some rocks, such as the Lias Coal-measure, &c, fossils are per- 

 fectly mineralized ; while in others, such as the Cretaceous, they 

 are not only mineralized, but have formed a nucleus round which 

 foreign matter has accumulated. 



If Oldhamia is a mineral structure, why does it occur under such 

 peculiar circumstances ? It has only been found in the Irish 

 Cambrians, and in them it is confined to very limited strata ; often 

 scarcely the thickness of your nail, so that only those acquainted 

 with its habitat can find it. 1 If it is a mineral, why does it not 

 also occur in the exactly similar rocks adjoining these thin seams 

 or layer, the layer or seam being identical in composition with the 

 associated rocks ? 



Exactly similar rocks to those at Bray Head, in which the 

 Oldhamia are found, occur in the Cambrians of other places in 

 Ireland, and in those of England and America ; yet in none of 

 these places has it been found. Dendrites occur everywhere if the 

 rocks are similar : why, therefore, if Oldhamia is a mineral, is it not 

 similarly distributed ? 



Also in various places inside and outside Ireland there are 

 rocks all made of identical mineral constituents to those of the Irish 

 Cambrians in which the Oldhamia is found. The seare of Silurian, 

 Ordovician, and Cambrian ages, and in none of them, except those 

 of proved Cambrian age, has the Oldhamia been found. If it is a 



1 The late Mr. E. Leeson, Fossil Collector on the Government Survey, and myself 

 collected boxes of Oldhamia at Bannow ; yet no one since, except Mr. Clarke, seems to 

 be able to find it. At Cahore bathing-place Messrs. Baily, Leeson, and myself, carried 

 away a bag full of fossils ; yet twice since I was there, and I could not find the bed, 

 although I know the exact place where it is. Similarly, Flanagan's original station 

 on Carriek mountain was not known till a few years ago, when it was accidentally 

 discovered. 



