AMORPHOUS OR ERUPTIVE TRAP — ALTERED ROCKS, ETC. 



273 



in all strata which have been equably deposited and symmetrically consolidated, that no 

 doubt can remain as to the nature of their origin. In these we find numerous joints, 

 diagonal, transverse, and longitudinal, having as precise relations to the planes of the 

 beds as any joints in sandstone, limestone, or shale; their course changing uniformly 

 with every variation in the strike and inclination of the strata. This point of coinci- 

 dence between the bedded trap rocks and the ordinary accumulations of sediment is 

 the more essential to keep in view, because the same symmetry is not observable in the 

 intrusive trap rocks 1 . 



" Chief masses of Eruptive or Amorphous Trap. — Altered Rocks, fyc." 



We now pass to the consideration of the principal masses of unbedded or eruptive 

 trap, which have dislocated at various points the sedimentary deposits and volcanic 

 grits, and thrown them into anticlinal and synclinal lines. 



The structure of the chief mountain (Corndon) is best seen in its east and south-eastern 

 flanks in bold and rugged escarpments, large subsided masses of which encumber the 

 sides of the Hyssington Marsh. The rock is chiefly a coarse-grained amorphous green- 

 stone, of equal parts of compact, light-coloured felspar and dark hornblende, with small 

 crystals of iron pyrites; but in parts it passes into a finely granular basaltic rock. 



If this be considered the principal axis of this convulsed region, we find that on both 

 sides of it there are other courses of eruption, producing parallel anticlinal lines of greater 

 or less longitudinal extension. Any transverse section from the western or north- 

 western flanks of the district to the Stiper Stones would pass over these separate anticlinal 

 lines, the strata on each side having reversed dips. These lines are in general directly 

 upon the prolongation of short ridges of intrusive trap. (See sections, PI. 32. figs. 1, 2 

 and 3.) 



A transverse section from the neighbourhood of Woferton, Marton, or Wilmington, to any point 

 along the chief axis, not only exhibits a succession of fossiliferous strata as before described with 

 much volcanic grit, but also with unconformable protruded trap. Some of the most prominent 

 of the exterior elevations of trap occupy the " ridge," extending from the flanks of Marrington 

 Dingle to the north-east, through the Black Knolls to the Crest Hills. (PL 32. f. 1.) This ridge 

 has a length of about three and a half miles, and an average breadth of a quarter to half a mile, 

 except at the spot called -the "Black Knolls," where various small promontories of trap give 

 it a width of nearly one mile. Within this space the trap protrudes at many parts through the 

 shale. At Wotherton quarries (the westerly end of the trap) the nucleus of the hill is concretionary 

 felspar, passing into a greenstone c, which on two sides graduates into bedded felspar b and on a 

 third throws off shale a in dislocated forms, containing a vein of sulphate of barytes thirty feet 

 wide d, from which the mineral is extracted for use. 



1 See chapter 20, in which the subject of jointed structure is considered. 



