458 



LITHOLOGICAL CHARACTER OF THE TRAP. 



which protrude through and dislocate the overlying strata ; sometimes throwing them 

 off in discordant directions, at other times enveloping their fractured and dismembered 

 portions within the masses of the trap. But while I differ from Mr. Weaver in my 

 opinion of the origin of these rocks, 1 have sincere pleasure in quoting his lucid mine- 

 ralogical description of them. 



"The trap is of a very variable character in different quarters, consisting of granular and compact 

 greenstone (the former seldom appearing of a distinctly crystallized structure), with occasionally 

 disseminated portions of hornblende or augite, sometimes, though very rarely, graduating into 

 basalt, of granular and compact felspar, of claystone and amygdaloid, all of which, being mu- 

 tually intermixed, frequently interchange characters and pass into each other ; and hence the shades 

 of colour are never constant in any considerable portion of the mass, but fluctuate from black to 

 green, red, brown and grey, the predominant colours being reddish brown and grey. The rock 

 sometimes, also, though very seldom, assumes a porphyritic appearance, thinly scattered acicular 

 crystals of glassy felspar occurring in its substance ; but in general the common aspect of the trap 

 may be said to be that of a compact rather than of a crystalline or even subcrystalline production, 

 and where stealitic matter prevails, it is of a loose consistence, friable and earthy. It frequently 

 contains compressed ovoidal and spheroidal nodules and kernels of chlorite, green earth, calcareous 

 spar, brown spar and quartz, also balls of chalcedony and agate, the internal cavities of which are 

 sometimes lined with crystals of amethyst ; and when the more perishable ingredients are removed 

 by decomposition they leave empty cavities, whence the rock acquires a vesicular and scorious 

 aspect. Sulphate of strontian, sulphate of barytes, and prehnite, appear more rarely in the trap ; 

 which sometimes also includes portions approaching to compact brown iron stone, and brown jasper. 

 Veins, composed of carbonate of lime and brown spar, either pure or mixed with trap and chloritic 

 and steatitic laminae, not unfrequently traverse the rock, occupying the cross fissures, which in some 

 places divide it into cuboidal and other quadrangular concretions. These concretions sometimes 

 exhibit a tendency to exfoliation, disclosing concentric lamellar layers that surround a spherical 

 nucleus. The structure of the same mass of trap varies much in the course of its extent ; it is most 

 frequently amorphous, or irregularly divided by fissures ; but when adjacent to the interstratified 

 beds of sandstone, slate clay, and limestone, a faint tendency toward a corresponding division into 

 strata may be partially observed ; while in some other quarters, thin strata, from two to four inches 

 thick, may be casually remarked, singularly contorted and inflected, yet subdivided by cross joints 

 into rhomboidal prismatic concretions." (Geol. Trans, vol. i. p. 326.) 



To this lithological description I have only to add the occasional presence of serpen- 

 tine, which in their films (specially seen in the great quarries of Damory Mill) , cover 

 the faces of many of the bulging concretions, particularly near their points of contact 

 with the Caradoc Sandstone. This phenomenon, it will be recollected, has been pointed 

 out in various other localities under similar conditions, and notably in that case where 

 the old Radnor trap rocks, (p. 320.), burst through limestone and sandstone of the 

 Silurian System. 



But to determine the essential question, whether the trap rocks of this district have 

 been formed contemporaneously with, or posteriorly to, the strata with which they are 

 intermingled ? The points at which these rocks appear at the surface, are Middle Mill 



