56 STRATIGRAPHICAL GEOLOGY 



superposition, for their base is not seen and the oldest rocks which 

 overlie them are Silurian. Outside this central area are small 

 tracts of other rocks which are demonstrably pre-Cambrian, but they 

 are chiefly of volcanic origin while those of the Longmynd are 

 sedimentary. Lastly, there is a small faulted inlier of micaceous 

 schist near Rushtoii which may be older than any of the others. 



The volcanic series is known as the Uriconian, from the hill 

 called the Wrekin and the Roman town of Uriconium at its foot. 

 The mass of the Wrekin ridge is made up of a succession of 

 rhyolitic lavas with interbedded bands of ash, grit, conglomerate, 

 and agglomerate. Midway along the ridge there is an intrusive 

 mass of dolerite, and at each end (i.e. on Ercal Hill and Primrose 

 Hill) are patches of granitoid rock which may be intrusive or may 

 belong to an older series. This uncertainty is due to the fact that 

 the junctions appear to be faults, 23 but pebbles in a conglomerate 

 near the Wrekin prove that the rocks of the surrounding country 

 at the time of its formation consisted principally of (1) a coarse 

 granitoid gneiss like that of the Wrekin, (2) crystalline schists, 

 both quartzose and micaceous, and (3) grits which have clearly been 

 part of a sedimentary series. 



Rocks similar to those of the Wrekin occur in the Wrockwardine 

 Hills to the north-west, as well as in the Caradoc and Cardington 

 Hills near Church Stretton, and this volcanic series everywhere 

 strikes nearly east and west, while the Longmyndian and Cambrian 

 rocks strike north-east and south-west (see map, Fig. 21). On the 

 western side of the Longmynd, Pontesford Hill is largely made up 

 of rhyolitic lavas like those of the Wrekin, with intrusive dolerites. 



The Longmyndian or sedimentary series of the Longmynd is 

 divisible into an eastern and western portion. The eastern part 

 appears to be the older, and if there is no reduplication of beds 

 the succession is as follows (see Fig. 7), in descending order : 



5. Greenish slates weathering purple, with some grits. 



4. Hard greywacke (indurated sandy beds). 



3. Purple slates. 



2. Banded slates and greywackes. 



1. Soft dark shales. 



The apparent thickness of these rocks is over 15,000 feet, and 

 they dip regularly to the north-west, the annelid markings and 

 rain-spots on their upper surfaces proving this to be the true dip, 

 and that we follow an upward succession in going from east to 

 west. No definite fossils have been obtained except some im- 

 pressions, which may be those of a Lingulella. 



Passing westward this series is succeeded by another set of beds, 

 consisting (1) of reddish felspathic grits with a thick band of con- 



