500 RUM. MINERALS. 



veins of various sizes, being at times imbedded in an 

 amygdaloid, and at others in a basalt. 



Nodules of pale onyx agate are also found among the 

 substances which fall from Scuir more, but they are neither 

 abundant nor remarkable for their beauty, except where, as 

 sometimes happens, they are partially stained with green. 

 Among them I procured one specimen of an uncommon 

 appearance, consisting of fragments of highly brilliant 

 charcoal involved in white chalcedony; a portion, pro- 

 bably, of some mass of imperfectly silicified wood. The 

 aspect of this charcoal is peculiar, since it has the 

 lustre and apparent compactness of jet in the longitudinal 

 fracture, while it is so soft as to yield to the nail ; but 

 its fibrous texture and ligneous origin are easily traced 

 in the cross fracture v 



Another specimen is of considerable interest, as it 

 throws light on some disputed questions relating to 

 the origin of the agate nodules which occur in trap. 

 It is scarcely necessary to say that these have been 

 attributed to two distinct causes; the one a secretion 

 of the siliceous matter during the hardening of the trap 

 from fusion, and the other a posterior infiltration of a 

 watery solution of silica in cavities previously existing 

 in the rock. If the former cause be considered the 

 true one, it necessarily follows that the rocks which 

 contain them have been in fusion. But if the latter 

 be supposed their true origin, it does not by any means 

 follow that these rocks are the result of a deposition 

 from water. The argument therefore which may be de- 

 duced from these nodules, on either hand, proves nothing 

 with regard to the containing rock, since, whatever its 

 origin may be, it will appear, from the consideration of 

 this and other phenomena, that the nodules are in many, 

 if not in all cases, posterior to the rock, and formed 

 by a watery infiltration of the substances which they 

 contain. Still it must be remarked, that the existence 

 of the original cavities in the trap is conceivable only 

 on the supposition of their igneous origin. 



i 



