502 RUM. MINERALS. 



by calcareous spar, so as partly to involve the stalactite. 

 It is obvious, from considering the structure of this 

 nodule, that, as in the cases occurring at Kinnoul, the 

 stalactite has been first formed in the cavity, and that 

 by a subsequent infiltration of carbonate of lime, the 

 calcareous spar has crystallized within the remaining 

 vacancy. The joint existence therefore of chalcedony 

 and calcareous spar in a nodule, must not be considered 

 as an instance of the separation of the one substance from 

 the other during the act of cooling, but as arising from 

 the subsequent infiltration of one substance into a cavity 

 partly occupied by another. The relative times of these 

 infiltrations will account for the different relative positions 

 occasionally occupied by the two substances, as well 

 as for the circumstance of their alternately impressing 

 each other ; while the cessation of the process before 

 the cavity was filled, will explain how the calcareous 

 spar sometimes possesses a geometric external form, when 

 at others it is deprived of that. It is no objection to this 

 view of the case, that the calcareous crystal is often 

 entirely surrounded by the chalcedony, since a soft 

 state of this substance, admitting the transudation of 

 water, must have equally existed to admit the formation 

 of the siliceous stalactite itself, and since it is known 

 that common flint when first removed from its native 

 bed permits water to exude from its surface. In the 

 instance also of quartz veins, as well as in the calcareous 

 veins of schist, abundant examples occur of their formation 

 in hard rock, in consequence of watery infiltration. It 

 need scarcely be added, that the existence of water in 

 similar agates, a fact mentioned by Dolomieu and others, 

 is easily explained on this view. 



The abundance of augit in this island would lead 

 us to expect specimens of it in distinct and independent 

 forms, but I did not meet with any ; no cavities among 

 the rock, in which only we can expect to meet with 

 such crystals, having occurred. The largest crystals 



