THE BASAL ROCK-GROUPS OF SHROPSHIRE. 123 



and the cracks are injected with red felspar. No igneous rocks are 

 seen near, but the alteration-effects just described are similar to 

 those observed at the contact of slate* with dolerite east of llagleth 

 Hill (p. 111). About 70 yards farther on, we come to slaty and 

 o-ritty rocks of a Uriconian appearance, which obviously could not 

 have produced the alteration. The most probable explanation of 

 the facts is a fault, with an intrusion of the dolerite which occurs in 

 force on the same line of fault farther south. There is very little 

 discordance of strike between the two systems at this locality. 



This fault does not, of course, prove an unconformity ; but it helps 

 to invalidate the hypothesis of conformity. 



3. Differences in the Conditions of Deposit. 



The Uriconian is essentially a volcanic formation. In the 

 Wrekin area, the rhy elites and felsite-grits form the chief mass ; 

 beds with rounded fragments being very inconspicuous. In the 

 Church-Stretton district, sedimentary matter, chiefly in the form of 

 volcanic mud, is more prominent. But, throughout the Uriconian 

 area, the comparative rarity of clear stratification is a marked 

 feature. The ashes and hornstone of Lawrence-Hill Quarry, taken 

 as the typical Uriconian section, were described by Sir R. Murchison 

 and the Survey as intrusive " greenstone." Even the clear sedi- 

 mentation of Charlton Hill was overlooked, and the mass appears on 

 the map as " greenstone." Large parts of the Wrekin have been 

 subjected to close and repeated examinations without yielding any 

 evidence of bedding. The conglomerates of the Wrekin area can 

 hardly be called beds at all ; they are mere irregular patches, some- 

 what elongated in the direction of the strike of the ash-beds with 

 which they are associated, and in a greater or less degree they 

 shade off into the finer materials. 



In the Uriconian hills of Church Sfcretton, although I have observed 

 orits in thirty or forty different localities, in only four or five of them 

 has a strike been detected, and in only two of these cases could it 

 be proved to extend for many yards. The strikes are more per- 

 sistent in the halleflinta, but a glance at the map reveals how rarely 

 they can be followed for great distances. The irregularity in the 

 shape of the masses of grit in this area is a marked feature. One 

 example will serve as a type. It occurs at Hazier, and forms part 

 of the Uriconian rock lying east of the fault (p. 122). When I 

 first approached this spot, I saw what looked like a granitic rock 

 intruding into slates. The seeming granite was without trace of 

 bedding or lamination. It cropped up in the road in irregular 

 masses amidst laminated slaty rocks ; on the northern bank it rose 

 up into the slate in a somewhat dome-like form, and on the southern 

 side it also appeared irregularly. A slight examination with a lens 

 dispelled the illusion, and proved that the rock was a quartz -felspar 

 grit. These irregular lumps of grit are explicable only as the 

 result of direct volcanic action. 



Contrast the irregular arrangement of the volcanic ejcctamenta of 



