458

LITHOLOGICAL CHARACTER OF THE TRAP.

which protrude through and dislocate the overlying strata; sometimes throwing themoff in discordant directions, at other times enveloping their fractured and dismemberedportions within the masses of the trap. But while I differ from Mr. Weaver in myopinion of the origin of these rocks, I 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 compactgreenstone (the former seldom appearing of a distinctly crystallized structure), with occasionallydisseminated portions of hornblende or augite, sometimes, though very rarely, graduating intobasalt, 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 shadesof colour are never constant in any considerable portion of the mass, but fluctuate from black togreen, red, brown and grey, the predominant colours being reddish brown and grey. The rocksometimes, also, though very seldom, assumes a porphyritic appearance, thinly scattered acicularcrystals of glassy felspar occurring in its substance; but in general the common aspect of the trapmay 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 frequentlycontains compressed ovoidal and spheroidal nodules and kernels of chlorite, green earth, calcareousspar, brown spar and quartz, also balls of chalcedony and agate, the internal cavities of which aresometimes lined with crystals of amethyst; and when the more perishable ingredients are removedby decomposition they leave empty cavities, whence the rock acquires a vesicular and scoriousaspect. 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 chloriticand steatitic laminae, not unfrequently traverse the rock, occupying the cross fissures, which in someplaces divide it into cuboidal and other quadrangular concretions. These concretions sometimesexhibit a tendency to exfoliation, disclosing concentric lamellar layers that surround a sphericalnucleus. The structure of the same mass of trap varies much in the course of its extent; it is mostfrequently amorphous, or irregularly divided by fissures; but when adjacent to the interstratifiedbeds of sandstone, slate clay, and limestone, a faint tendency toward a corresponding division intostrata may be partially observed; while in some other quarters, thin strata, from two to four inchesthick, may be casually remarked, singularly contorted and inflected, yet subdivided by cross jointsinto rhomboidal prismatic concretions.” (Geol . Trans. voJ. 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), coverthe faces of many of the bulging concretions, particularly near their points of contactwith the Caradoc Sandstone. This phenomenon, it will be recollected, has been pointedout in various other localities under similar conditions, and notably in that case wherethe old Radnor trap rocks, (p. 320.), burst through limestone and sandstone of theSilurian System.

But to determine the essential question, whether the trap rocks of this district havebeen formed contemporaneously with, or posteriorly to, the strata with which they areintermingled ? The points at which these rocks appear at the surface, are Middle Mill