RUM. MINERALS. 501 



It was formerly shown in a paper on the hill of 

 Kinnoul,* that agate nodules are sometimes found con- 

 taining cavities in which stalactites of chalcedony depend 

 from the upper part ; and that in other instances, of 

 which Faroe produces numerous examples, there is found 

 a distinct stratum of horizontal and laminar chalcedony 

 filling the lower side of the cavity; sometimes rising 

 in its progress so as to entangle the dependent stalac- 

 tite, and finally in certain cases involving the whole. 

 It can scarcely admit of a doubt that the siliceous matter 

 has in these instances been deposited in a gradual manner 

 from a watery solution, since in one or more of such 

 nodules all the stages of the progress can be traced. 

 In some, the whole cavity is filled with solid matter, 

 while in others the process appears to have prematurely 

 terminated, a cavity remaining. The causes of this are 

 to be sought for in some change of the rock itself, 

 refusing further passage to the silicated solution; and 

 however difficult it may be to explain this change, it 

 is by no means uncommon in the case of calcareous 

 infiltrations; which are sometimes found to have ceased 

 entirely, from the rock no longer admitting the trans- 

 udation of the calcareous solution. A fact of this 

 nature is pointed out under the head of Lismore. 



It is well known that calcareous spar is frequently 

 found in the centre of the agate nodule, sometimes 

 regularly crystallized within the unfilled cavity, and at 

 others occupying all that space which the chalcedony 

 has left. In some cases it impresses its form on the 

 chalcedony, while in others it is impressed by that 

 substance. The present specimen illustrates these ap- 

 pearances. It is a pale zoned agate (onyx), from one 

 side of which, to be considered doubtless as the upper, 

 stalactites of the same substance depend.f That part 

 of the nodule which is not siliceous is partially filled 



Geol. Trans, vol. iv. t Plate XXX. fig. 2. 



