4o'J PROF. W. S. BOULTOX OX THE [NOV. I9O4, 



rocks ' in the middle of the hill, with ' volcanic basic rocks ' and 

 ' crystalline basic rocks ' in amongst the latter. 



Thus it will be seen that Dr. Callaway regards Pontesford Hill 

 as made up in part of Archaean rhyolite and hornstone belonging to 

 his Uriconian Series, with intrusive basalt of post-Cambrian age. 

 the whole faulted against the Shineton (Upper Cambrian) Shales, 

 which occupy much of the valley between the Longmynd and Stipei 

 Stones. Mr. Blake, on the other hand, regards the hill as made up 

 of slates and grits of Upper Cambrian age, with two felsites and a 

 basic group, all of which are intrusive in these Cambrian rocks. 



My attention was first directed to the Pontesford district, when 

 assisting Prof. Lapworth in its mapping ; and in 1890 I commenced 

 the detailed study of its petrology. At that time Prof. Lapworth 

 had ascertained that the hill is practically made up of igneous 

 rocks, both bedded and intrusive ; and that there are two acid and 

 two basic groups present, the older basic group forming an inter- 

 bedded part of the local, so-called ' Uriconian ' volcanic group, while 

 the newer basic is intrusive in this older series. His microscopic 

 sections of some of the lower basic rocks had been identified as 

 palagonite-tuffs by Dr. Teall. As it appeared that most of the 

 lithological types of the so-called ' Uriconian ' Series of Shropshire 

 exist within the limits of the hill, Prof. Lapworth urged that I 

 should work the petrology of its rocks in detail as types for other 

 Shropshire areas ; and, as Pontesford Hill is isolated and circum- 

 scribed hy faults, such a study had also this further advantage, 

 that it did not involve the stratigraphical relation of its rocks to 

 those of the neighbouring Shropshire formations. 



II. General Structure or Pontesford Hill. 



I Map, PL XXXVIII, & Sections, PL XXXIX.) 



The hill is diamond-shaped in plan and bounded on all sides by 

 faults. Although, so far as I am aware, a boundary-fault is 

 actually visible at one place only (see p. 465 & fig. 3), the line of 

 the faults can be precisely traced : partly by the sudden change in 

 the slope of the ground, owing to the hard volcanic rocks of the hill 

 coming against the relatively -soft shales of the valley, and partly 

 by a line of springs, which occur at short intervals along the foot 

 of the hill. Along the western flank the rocks brought down against 

 the Pontesford volcanic rocks are dirty-green, or pale-buff, well- 

 laminated, shivery shales, exposed only in very few places. In 

 these I have so far failed to detect fossils, but they are classed 

 by Dr. Callaway as Shineton (Tremadoc) Shales. 1 At one place 

 (see p. 465) these shales are seen faulted against the andesite-tuffs 

 and intrusive dolerite of the hill ; while, in a stream-course a little 

 to the west, the same weathered shale is visible in situ, with loose 

 fragments of drifted fossiliferous Bala rock. Still farther west and 



l r< J 



1 Quart. Journ. Geol. Soc. vol. xxxviii (1882) pp. 121, 126. 



