BASAL CAMBRTAX ROCKS OF SHROPSHIRE. 413 



close at hand, but is not overlain by the grit. They may have once 

 been overlain by grit, but have afforded it a less secure foundation, 

 so that it has been more readily denuded. The absence of pebbles 

 of this rock in the Cambrian conglomerates is not conclusive, since 

 there is nothing in the mnjority of exposures that could produce a 

 pebble; and the crystalline portions may be of a different and later 

 date. We can thus get no nearer to the date of the basic rocks than 

 that they are anterior to the quartzite which lies upon them, and 

 that some of them, at least, are posterior to all the acid rocks. 

 Their position at the present moment in relation to the rhyolites is 

 very complicated : but this is due quite as much to later foldings 

 and squeezings, possibly subsequent to the formation of the quartzite, 

 as to the location of their, original eruptive centres. 



§ lY. Possible mork Ancient Hocks in the District. 



The rocks already dealt with form the principal portion of the 

 Pre-Cambrian rocks of the district : but there are two other groups 

 to which Dr. Callaway has already assigned a similar antiquity, viz. 

 the llushton schists and the gneiss of Primrose Hill (see fig. 8). 

 With regard to the former there is little or no stratigraphical evidence 

 available, and I have nothing to add to Dr. Callaway's petrographical 

 descriptions* ; I must only remark that the mica-schists and asso- 

 ciated rocks are more altered by pressure tlian any other rocks of 

 the neighbourhood, and call to mind many of the schists of the 

 eastern district in Anglesey. Such strike as they show is more or 

 less concordant with the strike of the Longmynd rocks ; but they 

 lie on the eastern side of the fault. It is very possible, considering 

 the great exposure of the quartzite and the higher elevation of the 

 Wrekin on the one side, and the coming in of the Trias on the 

 other, that the throw of the main fault may be increased before 

 reaching Rushton, and therefore that there may be a greater interval 

 between the lowest accessible rocks. The Rushton schists may 

 therefore be far lower in the series than the dark shales of the Long- 

 mynd border, and may actually rej)resent some of the rocks of 

 Anglesey, and this, at all events, is their most probable position ; and 

 if this correlation is correct, it would indicate a continuous deve- 

 lopment of the Monian series in the district. 



The second mass, the gneiss of Primrose Hill, is intimately as- 

 sociated with the eurite and is considered by Dr. Callaway as forming 

 part of the same group. This is not very likely to be the case if 

 the latter is an eruptive rock, as 1 believe it to be ; but independently 

 of this the mode of occurrence of the gneiss is very peculiar. 

 Towards the bottom of the western slope of the hill there is a small 

 mass of it not many yards square, and there is another narrow 

 patch of it, about 2 feet broad and a few yards in length, quite 

 close to the summit, and this is all that can be called true gneiss, 

 though some portion of the eurite is slightly gneissose. There is 



* Geol. Mag. 1884. 



