260 TRAP AND ALTERED CAMBRIAN ROCKS — LONGMYND, ETC. 



between the vertical and hardened beds of slaty greywacke, amid which they terminate upwards 

 thus, 



37. The trap rocks of the Longmynd, are principally light and dark-coloured green- 

 stone, composed of compact felspar and hornblende; the felspar of some varieties 

 being coloured green by chlorite. White calcareous spar, both in veins and thin 

 flakes, and crystals of iron pyrites are of frequent occurrence. In one of the 

 bosses near the summit of the mountain, the greenstone is highly crystalline, 

 and contains acicular white crystals of common felspar, in a base of pale green, 

 compact felspar. This rock occurs between two feeders of the brook, at about 

 two thirds of the ascent from Little Stretton to the summit, and is the best ex- 

 hibition of trap in the section, being about 100 feet high, and of forms ap- 

 proaching to columnar. The highest point of rock visible, beneath the thick covering of heather 

 on the summit, is a highly crystalline, dark-coloured greenstone, which juts out from amid purple- 

 coloured sandstone and conglomerate. This spot is about 100 feet beneath the pole placed by the 

 trigonometrical surveyors to mark the highest point. Another variety of trap, observable about 

 midway in this section, is a concretionary rock of dull flesh, and dark green compact felspar, having 

 the mottled aspect of some of the varieties of the Caradoc chain, and passing into a greenstone. At 

 a few hundred paces to the west of the pole cottage, or summit, I met with a large block of rock 

 differing from all those yet described; consisting of separate portions of pink and grey, compact 

 felspar and quartz, in a base of smaller grains of quartz mixed with felspar. At first sight, this 

 rock had all the appearance of one of the conglomerates which abound in this part of the mountain. 

 After all, is it not possible that the included portions of quartz may originally have been pebbles, 

 which being enveloped in a trappean matrix, have been so acted upon by heat, that all the lines of 

 separation between the fragments have been obliterated, and the whole have assumed the characters 

 of a true concretionary rock ? 



The prevailing strike of the vertical beds which constitute the chief masses of the Longmynd is 

 from north-east to south-west, but this deviation is subject to many partial variations amounting 

 sometimes to 25°. These aberrations seemed to me to be owing to the local protrusions of trap, 

 for on following the strata to some distance from that rock, the north-east and south-west strike 

 was found to be very persistent. 



The thin bands of felspar rocks, which alternate regularly with the beds of slate, have been al- 

 ready mentioned (p. 256.). They would seem to belong to that class of contemporaneous trap rocks, 

 of which, so many clear examples are adduced in the Silurian System, and though here too ill ex- 

 hibited to be much insisted upon, they are identical with rocks of that character which occur amid 

 the older slate, and to which allusion has been made (p. 76.). 



Hills of Ratlinghope. — The western and north-western slopes of the Longmynd consist of the 

 " compound sandstone," schist, and conglomerate, similar to those near the summit, and are here 

 and there intersected by masses of trap. Near the tops of the hills, north of Ratlinghope, some of 

 the latter jut out in hard pinnacles, and are of unquestionable trappean origin, being made up of 

 compact felspar and minute grains of hornblende, but they pass into other rocks, which appear to be 

 the prevailing coarse grits of the mountain in an altered condition with included fragments of in- 

 durated purple schist or slate. All traces of stratification are lost in these indurated masses, some 

 of which contain veins and druses lined with crystals of quartz, their surfaces being occasionally 

 coated with bitumen or mineral pitch. Besides the rocks of compact felspar, and the associated 



