THE BA8AL ROCK-GROUPS OP SHROPSHIRE. Ill 



The only locality in this area where intrusion appears to be 

 affirmed is Helmeth Hill. Here the " boundary of the slate " is 

 said to " zigzag amongst the igneous rocks." I searched this little 

 elevation from end to end, but could not find any slate near any 

 igneous' rock, except the ordinary dolerite, which, of course, is 

 unimportant. The summit-ridge is mainly made up of halleflinta 

 and oTit of ordinary Uriconian types, and I have seen no trace of 

 Longmvnd slate. The " rhyolite " which, according to Prof. Blake, 

 occupies the southern half of the ridge, is a good quartz and felspar 



grit ! 



Prof, Blake appears to have overlooked what I believe to be the 

 only locality in this area where a rock which may be Longmyndian 

 occurs in actual contact with an acidic igneous mass. This section 

 is briefly noticed in a former paper * of mine. The slate or shale, 

 which forms a very thin band, dips in a westerly direction in con- 

 formity with the apparently overlying Uriconian grits and argillites. 

 At its northern end it is surrounded by dolerite and has undergone 

 alteration, while its cracks are injected with red felspar. A few 

 yards farther south, the slate is immediately succeeded by a band of 

 felsite, which is overlain by dolerite. The felsite is of a brick-red 

 hue, and closelj'^ resembles a felsite which in Charlton Hill forms 

 a d5'ke, cutting across the strike of the Uriconian. A slide from 

 each locality has been microscopically examined. Prof. Bonney, 

 who favoured me with his assistance in my earlier work in 

 Shropshire, has kindly looked through the slides prepared for this 

 paper, and given me his opinion on critical points. The slide from 

 the Charlton-Hill dyke he describes as a " quartz-felsite, showing 

 some approach to a microgranophyric structure.'' The other has a 

 " somewhat similar structure, but less strongly marked." He 

 agrees with me that these felsites are " rather different from the 

 ordinary Wrekin lavas." It is probable, therefore, that the Hag- 

 leth felsite, like the mass at Charlton Hill, is an intrusive dyke. 

 A fault, shown on the Survey Map, passes along the base of the 

 Ragleth where the slate, felsite, and dolerite appear, and it is 

 possible that the slate is a faulted fragment of the Longmyndian. 



(b) Pontesford-Hill Area. — This is the only locality where it 

 is detinitel)'' asserted that contact-alteration has been produced by 

 rhyolite alleged by me to be Uriconian. At the bottom of p. ^02 f, 

 it IS stated that " the slates and grits which here alternate above 

 he falls are very much altered as they approach the igneous rock, 

 the grit being rendered micaceous and the slate chiastolized, and 

 both are indurated." The two formations, Longmyndian and 

 Uriconian, are clearly exposed in a stream-section, and the exact 

 spot of the supposed metamorphism is fixed by the words " above 

 the falls." jS"o signs of alteration are, however, seen on the ground, 

 nor can the grit and the shaly rock be distinguished in hand- 

 specimens from typical Longmyndian. Two specimens were selected 

 for the microscope as being rather more shaly than usual, and 



* Quart. Journ. Geol. See. vol. xxxv. (1879) p. 659, 

 t Ibid. vol. xlvi. (1890). 



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