Mr. Murchison on the Transition Rocks of Shropshire, S,c. SI 3 



geographical relations to the ovoidal mass, as the tail of a school- 

 boy's kite does to its body. This phenomenon he will hereafter 

 point out as occurring in other valleys of elevation of similar 

 epoch. 



This line of elevation trending from north-west to south-east, is 

 stated to be prolonged through May and Huntley Hills to Flaxley in 

 Gloucestershire, where the three superior grauwacke formations 

 being reproduced, a separate description of them is annexed. 



Mr. Maclauchlan had previously laid down upon the map of the 

 Ordnance Survey a correct outline of this mass of transition rocks, 

 in its general relations to the old red sandstone. 



IV. Lines of Direction and Dislocations. — The author here referred 

 to the sheets of the Ordnance Survey on which he had laid down 

 numerous details illustrative of the outlines of each formation, and 

 of the direction and dislocation of the strata. 



The prevailing strike of all the deposits described, is from 

 north-east to south-west, as indicated by the line of junction of the 

 Ludlow rocks with the old red sandstone in the counties of Salop, 

 Hereford, Brecknock and Caermarthen. The western limits of these 

 counties, together with those of Montgomery and Radnorshires, ex- 

 hibit the same direction of the strata. This tract is about 100 miles 

 in length from Lilleshall Hill and Wellington on the north-east to 

 the mouth of the Towey on the south-west, and has a breadth of 

 from 30 to 40 miles. Within this space there are numerous minor 

 axes of elevation, which are only traceable for short distances on the 

 strike, but they are all parallel to each other, and subordinate to 

 the same great line of elevatory movement. These are for the most 

 part marked by eruptive ridges of trap rock, which tilt the strata 

 upon their flanks both to the north-west and to the south-east ; 

 and wherever such parallel outbursts are numerous, as between 

 the Wenlock edge and the river Vierniew in Montgomeryshire, 

 they occasion the folding over and repetition of strata of the same 

 age, far to the north-west of their regular line of bearing, upon the 

 confines of Shropshire and Herefordshire. Wherever on the con- 

 trary, the longitudinal influence of these short outbursts has ceased, 

 the younger formations, or those of Ludlow and Wenlock, overlie 

 in slightly disturbed positions, the vertical and dislocated beds of 

 the older groups. Hence it is, that the old red sandstone and the 

 Ludlow rocks occupy so large and detached an area on the west, as 

 in Clun, Knighton and Radnor forests. 



The line of elevation of which the trap rocks of Old Radnor 

 mark the center, terminates on the north-east in the valley of ele- 

 vation of Wigmore, having the promontory of Ludlow as its apex; 

 and on the south-west in the narrow anticlinal ridge of Corn-y-fan 

 near Brecknock. The trap rocks extending from Llandrindod to 

 Builth, and other ridges of porphyritic rocks discovered by the 

 author in Brecknock and Carmarthenshire's (hereafter to be de- 

 scribedj, occasion similar, short anticlinal lines all trending from 

 north-east to south-west. This general line of direction is broken 

 through transversely by many cracks and fissures, some of which 



