236 



PROP. S. H. REYNOLDS ON A SILURIAN [May I907. 



Farm — have the three types of igneous rock been met with in regular 

 series as one passes across the igneous band; — the coarse ashy 

 conglomerate is exposed by a Utile pond just south of the main 

 road, and the trap and fossiliferous tuff were proved in trenches. 

 The probable relation of the three rocks is seen in the appended 

 section (fig. 8). The fact of this succession not having been observed 



Pig. 8. — Section a quarter of a mile south-west of Tadhill Farm. 

 N.N.W. S.S.E. 



Little pond south 

 of main road 



Trench 



o?o-°;-?o?6 



.t°-- ?o.o 



•.OO..O" 

 .■O'aT.O"-'.* 



s ?;.°.:2.v 



Coarse ashy 

 Conglomerate 



X X X x x X 



XX X X X X X 



* * * * XXX. 



x < X X X X X X 



X AX X X X X X X 



xxxxxxxxx 



X XX X X XX X XXX 



Trap (pyroxene- 

 andesite) 



Fine tuff with 

 Silurian fossils 



[Horizontal scale : 3 inches — about 2U0 yards.J 



elsewhere in the district, would be readily explicable by the badness 

 of the exposures and by the overlap of the Old Red Sandstone. 



The extremely-irregular character of the coarse ashy con- 

 glomerate and the absence of any sign of stratification tell against 

 this view. 



(3) That the coarse ashy conglomerate may be an old river- 

 gravel, deposited during a terrestrial period subsequent to the 

 fossiliferous Silurian and prior to the Old Ked Sandstone of the 

 district. — A deposit of this character from Glen Brittle House 

 (Skye) is described by Mr. Harker, 1 and agrees closely with the 

 Mendip rock : (a) in the rounded character of the blocks ; (b) in 

 the occurrence of sedimentary material as well as material derived 

 from the associated lavas ; (c) in the fact that the conglomerate 

 sometimes passes into a tuff devoid of pebbly material; and (d) in 

 the ashy character of the matrix. Mr. Harker writes of my 

 specimens, 



' The specimens are remarkably like the fluviatile volcanic conglomerates of 

 Skye, Rum, and Mull.' 



The patchy distribution of the Mendip rock would agree well 

 with its being a fluviatile deposit. On the other hand, the 

 microscopical characters of the matrix are rather those of an 

 original tuff than of a tuff re-sorted and re-deposited by water- 

 action. A further point of difference between the Mendips and the 

 Western Highlands of Scotland is that, while in the latter region the 

 igneous series as a whole is terrestrial, in the Mendips the igneous 

 series includes marine fossils. 



(4) That the coarse ashy conglomerate may represent the necks 

 of the volcanoes from which the trap and normal tuff were ejected. — 

 The uniformity in texture of the rounded trap-blocks throughout 



1 • The Tertiary Igneous Kocks of Skye' Mem. Greol. Suit. 1904, p. 25. 



