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ON THE GEOLOGICAL POSITION OF THE CRYSTALLINE 

 ROCKS OF THE MALVERN HILLS. 



By n.VllVEY B. HOLL, M. D., F. O. S., &c. 



The oldest rocks in the district are those which, with their granite beds 

 and their trap dykes, form the hills. They consist of various kinds of gneiss, 

 which is sometimes in thick massive veins, at others fine-grained and thinly 

 bedded ; and interstratified with these are other crystalline and semi-crystal- 

 line rocks, as hornblend schist, raica schist, felstone, syenite, and granite. 

 Some of the latter may perhaps be intrusive, but this is very doubtful, and for 

 the most part, like the syenite, it is a derivative rock. 



These rocks were all of them in existence before the period of the 

 Hollybush Sandstone, as shown by their infra-position ; for the sandstone 

 rests upon them unconformably, and is not penetrated or altered by the trap 

 dykes, and as the age of this sandstone is known to correspond with the 

 Lingula Flags of Wales, it serves as the point of departure in our attempt 

 to determine the geological position of the underlying rocks. 



The relative position of the Hollybush Sandstone to the metamorphic 

 rocks, resting as it does upon their upturned edges, indicates that the 

 latter had undergone disturbance before the sandstones were deposited ; and 

 that the intrusion of the trap rocks was posterior to this uplifting is shown 

 by the manner in which the continuity of the bedding of the gneiss, and 

 especially the granite veins, has been interrupted by their outburst, and also by 

 the .different appearance which the trap rocks present at their margins from 

 what they do in the interior of their mass, the more rapid cooling of their 

 circumferential portions having rendered them less highly crystalline at their 

 edges than they are in their interior ; and from this we may also infer that 

 the metamorphic action had long ceased, and that the gneissic rocks were no 

 longer at an elevated temperature at the time of their eruption. 



From other evidence, derived partly from the manner in which the suc- 

 cessive beds of the superimposed Hollybush sandstone overlap the lower ones, 

 and creep as it were up the sides of the hills, and mantle into the hollows 

 between them ; and from the pebbles of gneiss, &e., contained in their 

 lowermost beds, we learn that the crystalline rocks were elevated above 

 the sea level and subjected to denuding influences which formed them into 

 a mountain ridge before the sandstones were deposited. Now, as all these 

 events occurred previous to the period of the Hollybush sandstone, or 



