EXTINCT VOLCANO IN. EKITAIN. 1 &KL 



Cader Idrie mountain, which have evidently taken place. 

 But with respect to the crater itself, this appears very clearly 

 to have derived its origin from the violence of an explosion 

 upwards, in which a very considerable portion of the highest 

 eminence was torn from its native bed of rocks, and thrown 

 to a considerable height over the other parts of the moun- 

 tain. In confirmation of this suggestion it should be men- Proof* of this, 

 tioned, that the summit of the mountain is covered with an 

 immense wreck of the stones, ejected as it is presumed from 

 the crater at the time of this explosion ; it would be difficult 

 otherwise to account for the vast profusion of tljose stones 

 scattered in all- directions about the loftiest elevations, and 

 which, from the confused manner 'in which they are dis- 

 persed, must have been thrown into iheir present situation by 

 no small violence. Myriads of these stones have borne a 

 regular crystallized form, though from their great bulk and 

 weight they have for the most part suffered material injury 

 in the general convulsion. The usual length of these crys- Columnar 

 tals is from three to six or ten feet in length : some itte'&tire^^ feet long, 

 even fifteen or twenty, and one in : particular, which Mr. 

 Donovan has seen, was twenty-two feet three inches long* 

 They are however slender in proportion to the length. 



^The substance of these crystals is of the basalt kind, aud Basil tes. 

 corresponds very nearly with some varieties of the «' lave 

 porphyrc" of Etna described by Dolomieu, and Faujas de 

 St. Fond ; and in the form of its crystals agrees with others 

 of the basaltes prismatlque of the last author.. In the nep- 

 t urn ran theory it is not indeed admitted ,as a basalt, but as a 

 porphyry argil. It is the porphir-schtefcr of Werner, "ana 

 porphyry slate, or clinkstone porphyry of Jamieson. 



The suite of these stupendous crystals, which Mr. Dono- Specimens of 

 van collected from tb6 summit of Cader Idris last summer, 1 *™J^J ]?* 

 and has lately added to his museum, consists of a small tri- 

 hedral column about eighteen inches in length ;.a tetrahedral 

 column of much superior size ; an interesting portion of a 

 pentagonal column', and another of the san\e figure about 

 fwur feet in length, and having the termination of the crys- 

 tal complete. The latter is estimated at about five hundred 

 weight, but this is still exceeded by another of a somewhat 

 compressed hexagonal figure with an oblique termination. 



novatrs r 

 »eum. 



