424 Dr. BucKLAND on Agates in Dolomitic Strata, S^c. 



The theory of the formation of. agates in trap rocks by infiltration of silex 

 into their cavities is now generally admitted; similar infiltrations of chalce- 

 dony occur occasionally in other rocks, and in the cracks and cavities of sili- 

 cified organic remains. The finest examples of organic remains containing 

 agates are the silicified woods and corals from the tertiary strata of Antigua ; 

 in a less perfect state they occur in silicified wood of other formations. The 

 conversion of fossil shells to chalcedony and jasper is due to this same process 

 of infiltration. Remarkable examples occur in the green-sand formation of 

 Devonshire, in shells from the Whetstone pits at Blackdown*, which are 

 turned to limpid chalcedony ; and in the shells of the same green-sand at 

 Little Ilaldon Hill, near Dawlish, which are converted to red jasperj. 



I possess two agates from cavities of chert in the green-sand formation at 

 Lyme Regis, one having the structure of a box agate, the other of a fortifi- 

 cation agate ; and from the same green-sand I have an Echinus, the interior 

 of which is nearly filled with bluish chalcedony. Chalcedony also occa- 

 sionally assumes the form of agates, in veins and small cavities of primitive 

 and transition rocks ; and, indeed, wherever silex is present in a state of suf- 

 ficiently minute division to be filtrated into any small cavity, there the forma- 

 tion of agates may proceed. 



The occurrence of entire beds, as well as nodules of jasper and jasper- 

 agate in the mountains of dolomite near Palermo, which I agree w^ith Dr. 

 Daubeny in referring to our magnesian limestone formation, ailbrds a parallel 

 example of silex assuming the form of agates, jasper, and chalcedony in a for- 

 mation of the same age with that containing agates in the Mendip Hills. 



* In the collection of the present Dean of Bristol there is a specimen from Blackdown, in which 

 a small pond of stratified chalcedony occupies the cavity of one valve of a large Venus ; this 

 chalcedony is disposed in parallel and horizontal plates, like the plates at the bottom of a box 

 agate. The shell itself is converted to chalcedony, and mu.st have laid horizontally, when its ca- 

 vity received in a fluid form the silex, which is become an onyx composed of thin horizontal layers 

 of differently coloured chalcedony. 



•j- The fossils at this place afford the only example I ever saw, of shells converted into red 

 jasper: they are of the same species with those in the not far distant Whetstone Pits of Black- 

 down, between Honiton and Cullompton, which are usually in the state of light grey coloured 

 chalcedony. 



