ON Tin: ARCII.KAN ROCKS OF GREAT LIUTAIN. 



535 



pfrauitoid rock. This continaes for aboat a qaarfcci' of a mile, and is 

 then abruptly succeeded by rliyolite (the commencement of the main 

 mas)il/), the junction being probably a faulted one. It is possible that the 

 junction on the northern sido may also bo a fault, but to myself the 

 appearances appear much more favourable to the idea that the rhyolito 

 has broken through the granitoid rock. Tlio latter consists of quart/, 

 felspar (orthoclase, oligoclase ? and probably niicrocline ), with some iron 

 peroxide and a small quantity of a chloritic mineral. It is extremely 

 difficult to say whether it be a true granite or a granitoid gneiss (grani- 

 toiditc), but it seems impossible to doubt that it is a much more ancient 

 rock than the rhyolite. At Primrose Hill, at the southern end of the 

 range, the rhyolite again gives place to ' granitoidite ' of the Ercal Hill 

 type, with gneiss of Malvertiian type and a little diorite. 



Evldencvs of Geulocjical Aje. — Even if we consider the ' granitoidite ' 

 igneous, it must bo older than the rhyolitic; group, and we can hardly 

 hesitate to recognise in some of the Primrose Hill rooks a reappearance 

 of the ancient Malvernian gneisses. At any rate, the i-hyolitic group is 

 much earlier instead of later than the Caradoe, for Dr. Callaway has 

 shown that the quartzite which flanks the Wrekin is considerably older 

 than the HoUybush sandstone' (Lingula Flag), and it contains in parts 

 fragments of the rhyolitic rocks of the Wrekin. Hence the latter must, 

 at the latest, be Cambrian, or even older. 



The rhyolitic group reappears to the north-west of the "Wrekin, at a 

 distance of rather moio than a mile, in the neighbourhood of the village 

 of Wrockwardine ; and nor. r the southern end, at Lea Hock, there is a fine 

 mass of the ancient ' pitehstone,' exhibiting in parts beautiful perlitic 

 and spherulitic structui'es, which have been described and depicted by 

 .Mr. Allport. 



At Charlton Hill, along the same line to the south-west, there are 

 argillites, tuS's, and felstones, some of the latter being of the Wrekin type, 

 but one is porphyritic and apparejitly rather less acid, together with a very 

 interesting conglomerate, containing well rounded fragments of quartz, 

 felspar, gneiss, and various schists, indicating that tiie materials were 

 derived from a series of metamorphic rocks. 



Lilleshall Hill, a low ridge, about five miles to the north-east of the 

 Wrekin, consists of hard argillites, ashy slates, and rhyolitic agglome- 

 rates, with a small coulee (?) of rhyolite, the latter closely resembling 

 the same rock in the AVrekiu. Vverage dip 40° to NNW. 



There are exposures of rocks resembling the above described volcanic 

 ,i,n'oup in the district between the Wrekin and Caer Caradoc, at the latter 

 locality, at Hazier Hill, llaglett Hill, and near Hope Bowdler, and some 

 more west of the Longmynds,but as tliey arc inferior in interest to those 

 already described it may suffice to mention them.- 



(?.) The Liclieii Hills. — Tliis range forms the eastern boundaiy of 

 tile Severn valley, lying rather more than twenty miles north-east from the 

 Worcester Beacon (the culmhiating summit of the ^falvern chain) and 

 about the same distance south-east of the Wrekin. The principal rock is 

 a quartzite bearing .considerable resemblance to that which flanks the 

 Wrekin, the constituents of which have probably been derived from 

 granitoid rocks. Formerly tiv "■^ was regarded as altered Llandovei-y 



' Q. J. <:,'. .v.. vol. xxxi\. p. 7.*,4. 



■^ S<(0 Calliiway, /('.■. cit,, iiml \t'!. ::^:.\vii!. p. ll'j. 



