412 PROF. J. E. BLAKE ON THE MONIAN AND 



(see fig. 5). This ocenrs in isolated and irregular masses of consider- 

 able size, and might at first be thought to form only an integral portion 

 of the volcanic series of the hill. It is red in colour and has 

 abundant felspathic insets. There are three reasons for consi- 

 dering it intrusive. Pirst, the character of the rock is different 

 from any rock met with in the other volcanic hills, and is therefore 

 not an ordinary constituent of the series ; but it resembles more a 

 superficial form of the eurite, which has already been regarded as 

 intrusive, and, moreover, porphj'ries are more naturally intrusive 

 than more glassy rocks. Secondly, because in the Gaer Stones plan- 

 tation we find the ordinary grey and pink rhyolite becoming very 

 much discoloured on approaching it, turning to a bright red hue 

 owing to the oxidation of the iron. And thirdly, because one of the 

 masses of red grit already described, as seen in a crag on one side of a 

 valley, actually has a large vein of this porphyry intruded into its 

 midst (see fig. 9). If, therefore, the porphyry was of the same age 

 as the whole of the acid rocks of the hill, this grit would be earlier 

 in date than the hill on which it rests, which would be very difficult 

 to conceive of. 



Lastly we come to the basic rocks, which seem to be everywhere 

 present, mixed in the most intimate manner with the acid types. 



Fig. 9. — Porphyry intrusive in lied Grit at Cardington. 



There is seldom anything to indicate whether these are of later date 

 or contemporaneous with the acid rocks. They are usually less in 

 quantity than the latter ; but in the Lawley the proportion is reversed. 

 The great majority show no crystals, but are decayed into a dark 

 irregular mass. There are other associated rocks, as at the north 

 of the Lawley and in Hazier Hill, which appear to be the unstra- 

 tified ashy representatives of this kind of rock, such as I have called 

 pelite in Anglesey, and which would indicate that these basic rocks 

 are not in the form of intrusions, but are eruptive. If all the basic 

 rocks of the district are of the same age, they must be later than tlie 

 agglomerates of Lawrence Hill, and therefore one of the last to be 

 formed. Negative evidence to the same effect is afforded by the fact 

 that the conglomerates and grits, wherever known, always lie upon 

 the rhyolites and never on the basic rocks (unless Wartle Knowl be 

 an exception). Even at the spot where the porphyry intrudes in 

 the grit on Cardington Hill there is a crystalline dolerite which is 



