230 PROF. S. H. REYNOLDS ON A SILURIAN [May I907, 



opened 40 yards to the south of the target-pit again proved to 

 be in coarse ashy conglomerate, and showed that the rock in this 

 region extends over an area of 80 yards from north to south, and 

 that it probably occupies the whole space between the northern and 

 southern outcrops of the Old Eed Sandstone. A fifth trench was 

 dug on the slopes of the hill to the north ; but this, after passing 

 through 9 feet of Old Red Sandstone, was abandoned. 



III. The Petrographical Chaeacters oe tue Rocks. 

 (1) The Lava or Trap. 



Two specimens of the trap, one from Downhead and one from 

 near Stoke Lane, were submitted to the late Mr. Frank Rutley 

 and were described 2 by him. 



The Downhead rock he described as a compact dark-grey rock 

 showing crystals of hornblende in a hand-specimen, and under 

 the microscope octahedra of magnetite and crystals (probably) 

 of titaniferous iron. In the absence of opportunity for further 

 investigation he defined the rock as a felstone containing much 

 magnetite and a little hornblende. 



The Stoke-Lane rock he described in a hand-specimen as a 



' brownish-grey rock showing numerous minute greenish or colourless crystals 

 with a vitreous lustre.' (Op. cit. p. 208.) 



Under the microscope, the rock appeared to he rich in orthoclase 

 and plagioclase, and minute crystals of hornblende or augite could 

 be detected. He applied the term ' devitrified pitchstone-porphyry ' 

 to the rock. 



Sir Archibald Geikie and Dr. Strahan submitted specimens taken 

 from Beacon Hill and from Moon's-Hill Quarry to Dr. Teall, who 

 pronounced them to be undoubted andesites, his report being as 

 follows 2 : — 



' E. 3195 (11), Beacon Hill. 



' Conspicuous phenocrysts of plagioclase, and pseuclomorphs after [mis- 

 printed ' often ' in the original] a ferromagnesian constituent (augite or 

 enstatite), in a felsitic matrix containing traces of felspar-microlites. The 

 rock is too much altered for precise determination, but it undoubtedly belongs 

 to the andesites. 



' E. 3197 (13), Moon's-Hill Quarry. 



' A dark-coloured andesite composed of phenocrysts of plagioclase, augite, 

 and green serpentinous pseuclomorphs after enstatite (?), in a microlitic 

 groundmass.' 



The Moon's-Hill mass. — The Moon's-Hill rock being the 

 best exposed, the most accessible, and the most easily obtained in 

 a fresh state, may conveniently be described first. A thoroughly- 

 fresh specimen of the typical rock shows an abundant dark greyish- 

 purple matrix with numerous dark-green phenocrysts of pyroxene 



1 ' Geology of East Somerset & the Bristol Coalfields ' Mem. Geol. Surv. 1876, 

 App'.i, pp. 208-209. 



2 ' Summary of Progress of the Geological Survey for 1898 ' (1899) p. 111. 



