224 Dr. Mac Culloch on the Hill of Kinnoul. 



generally of a small size, and of a spheroidal or ellipsoidal form. 

 Their fracture is crystalline, and they are often perfectly transpa- 

 rent, but in all cases where they are in immediate contact with the 

 trap they possess no external crystallized form. It is only when 

 in contact with quartz or agate that they exhibit an external as well 

 as an internal appearance of crystallization. In other words, they 

 appear to bear the impression of the basalt when they are in im- 

 mediate contact with it, while on the contrary, when in contact 

 with quartz or agate, they impress their forms on these. Calcareous 

 spar thus assists in forming a compound nodule not uncommon in 

 these rocks. The spar is sometimes crystallized at liberty in a 

 cavity of quartz or agate ; at other times it is closely invested 

 by quartz, forming the center of a solid nodule, or is dispersed 

 in small crystals irregularly scattered through the whole pebble. 

 Both these substances occasionally retain their perfect characters, 

 even when in immediate contact ; while in other instances the 

 quartz for some small distance in the vicinity of the calcareous 

 crystal is converted into chert or into agate. 



Quartz is found among these rocks in the usual variety which it 

 exhibits when it is an inmate of trap. In its simplest form it is a 

 crust investing a cavity, and terminating interiorly in assemblages 

 of crystals of various sizes and of various colours, white, brown, 

 and amethystine. The exterior quartzy crust often puts on the 

 character of chert, sometimes that of chalcedony ; not unfrequently 

 the chalcedony appears in the form of a stalactite, of which the 

 several icicles are encrusted with beautiful and crowded assemblages 

 of crystals. These stalactites of crystals depend, as is usual in 

 stalactites, from the upper parts of the cavity. The nodules of 

 this description are of various magnitudes and often of a foot or 

 more in diameter, and, like the smaller ones, are shut in and 



