534 



iiKroKT — 1884. 





,: 



'II 

 ■' 



epidote are also present, the Ibrinor boing somtiLimcs abundant. Orthoclaso 

 and plagioclaso have been recognised among tbo felspars, and sometimes 

 the latter predominates ; the quantity of qnai'tz is veiy variable. Thero 

 are intrasivo diorites, and ])erhaps diabases, and ' granite-veins,' whicb, 

 however, are more probably ' segregation veins.' 



On the eastern side of tlio Herefordshire Bea()on, an area of rock 

 occars, forming a series of buttresses to the hill, whicli may possibly 

 belong to the latest part of the Archaean scries. The outci-ojjs occui' 

 over an area of about a mile from north to south, and half that distance 

 from east to west, and have been described by Dr. C. Callaway.' The 

 dominant rock is a very compact, flinty argillitc, of a greyish or reddisli- 

 grey colour, which has occasionally an ashy aspect. There is also some 

 of a felspathic rock, which appeared to me to be more probably a true 

 quartz-felsitc! and possibly intrusive. It is difficult to come to any conclu- 

 sion as to the diji and strike of the rocks, which are of a rather monotonous 

 and uninteresting character. They are certainly much newer than the 

 gneisses and schists of the main range, and are older than the Hollybush 

 sandstone, but their claim to be pre-Cambrian rests only on lithological 

 characteristics, which in this case, it must bo admitted, are not very con- 

 clusive. The Archa;an age of the gneissic series is now so generally 

 admitted that no proofs need be given in this summary. 



(4.) The Wrekin District. — This hill, so familiar to every Salopian, 

 forms a bold ridge, with the minor summits of the Ercal and Primrose 

 Hill at the northern and the southern end. On the Geological Survey map 

 the ridge is coloured as ' intrusive greenstone,' and surrounded with a 

 zone of altered Caradoc. Attention was first directed to the interesting 

 petrology of the district by Mr. 8. AUport,- and the stratigrajdiy of the 

 region was worked out by Dr. Callaway,'^ to whose paper is appended p. 

 note on the lithology of the district by myself. T)ie Wrekin is only 

 the loftiest of a series of outcropping ridges which extend interruptetUy 

 in a NE. to SW. direction from Lilleshall Hill to the south of the Caer 

 Caradoc chain — in all about twenty-nine miles from t!ie most extreme 

 points. It will be convenient to describe first the rocks of the Wrekin, 

 the most important portion of the chain. The Wrekin group, a line of 

 steep hills, is rather less than three miles long, and tlic area occupied by 

 the older rocks is at most about a third of a mile wide. The Wrekin 

 proper consists of a series of volcanic agglomerates and compact felstones. 

 We have in it, beyond all doubt, a portion of an ancient volcanic liill, 

 which has ejected rhyolitic lavas and scoi'ia; fluidal structure is common 

 in the fragments, spherulitic and perlitic structures are occasionally seen ; 

 the prevailing tint is a purplish red. Chemically the ' felstone' is almost 

 identical with the perlite of Schemnitz,' and, except that the rock i3 

 now deviti'ified, there is no reason why the names rhyolite and pitch- 

 stone should not be applied to some of its varieties. The fragments 

 included in the coarser agglomerates are often several inches in diameter. 

 There are indeed one or two small bosses of intrusive dolerite, Vut 

 the area occupied by them is so insignificant that it cannot justify th^ 

 designation of the whole massif as greenstone. The rhyolite is exposed 

 at the extreme north of the range (Ercal Hill), and is seen (in a large 

 quarry) to be overlain at a high angle by a coarse (rather decomposed) 



' Q. J. a. s., vol, xxxvi. p. -):](;. 



' V- 'f- f'- '^'•' vol. XXXV. p. M.i, 



- n. J. (,. S., vol. xxxiii. p 449. 



* AUport, {). J. (;. S., vol. xxxiii. p. ^tA 



