26 i Dr. Mac CuLLOCH on the Vitrified Forts of Scotland, 



ments of trap cemented by a whitish substance, which proves to be 

 the hard variety of calcareous spar, mixed with a sand of trap. 

 This trap sand is generally of a dark purple colour, resembling 

 many of the imbedded pebbles. Although this sand is the pre- 

 dominant ingredient in the paste, there are also found in it grains 

 of quartz, minute zeolites, garnets, crystals of calcareous spar, and 

 here and there prehnite, diallage, and chlorite slate, as far as it is 

 possible to speak decidedly of objects so very minute. The spar 

 which cements this sand into a common paste surrounds every 

 grain so as to form them into a perfect breccia, and enable the 

 whole to break with the splintery fracture above noticed, instead of 

 a granular one. Here and there the paste occupies large interstices 

 which have been formed by the approximation of two convex 

 surfaces of considerable extent, and from these it may be traced 

 insinuating itself through all the grains of the mass. It is evident 

 that the calcareous spar has been Introduced while in a state of 

 fluidity among the sand and gravel, as the larger pieces of paste 

 may be observed to envelope the grains of trap. Generally there- 

 fore we may consider the pudding-stone of Lorn as a congeries of 

 trap sand and trap pebbles, cemented by calcareous spar, a rock 

 often designated by the improper name of trap tufo. This how- 

 ever is not the place to enquire into the means by which the mass 

 was consolidated. That it is a case of an agglutinated rock differing 

 greatly from the ordinary sandstone breccia, or the ferruginous and 

 argillaceous pudding-stone, is very apparent. It resembles them 

 indeed only superficially and in its mechanical texture, and it will 

 be worthy the labour of geologists to direct their attention to the 

 pudding-stones of this coast with more care than they have 

 hitherto done. The other rocks are too well known to need any 

 description. 



