234 



PROF, S. H. REYNOLDS OjS t A STLURIAN [May 1907. 



IV. The Mutual Relations and Nature oe the Rocks. 



Of the four rock-types exposed in the area — Old Red Sandstone, 

 trap, normal tuff, and coarse ashy conglomerate — the only types 

 the mutual relationship of which is clearly seen in the field are 

 the trap and the normal tuff. The trap succeeds the normal tuff 

 with perfect conformity at Sunnyhill Quarry, the series dipping 

 north-north-westward at an angle of 40° to 45°. The occurrence 

 of tuff-debris at several points along the southern outcrop of the 

 trap is in accordance with the evidence at Sunnyhill, and indicates 

 the occurrence of a continuous band of tuff underlying the trap. 



In all probability, the Old Red Sandstone rests with discordance 

 upon the whole igneous series, as at one point or another it is seen 

 in close relation to each of the three members of the series. Thus, 

 at Beacon Hill, it is exposed on the hillside above the coarse ashy 

 conglomerate, and (if not faulted) its dip is such as to bring it over 

 the conglomerate (see fig. 6, below). 



Fig. 6. — Section passing southward from Beacon Plantation, 

 on tfie scale of 150 feet to the inch. 



Beacon Plantation 



N. 



Coarse ashy conglomerate 



At Downhead, on the other hand, it is seen on the slope of the 

 hill, while the fossiliferous tuff occurs a few yards down the slope 

 (see fig. 7, below). 



Fig. 7. — Section from Tadhill House to Bottlehead Springs, 

 on the scale of 6 inches to the mile. 



O.R.S, 



Trap (pyroxenerandesite) 



Fine tuff with 

 Silurian fossils 



At many localities there is evidence of its occurrence in rela- 

 tion to the trap. Such a discordant relationship is what would be 

 expected from the fact that there is no evidence of the presence of 

 the upper horizons of the Silurian Series. 



The possibility of the occurrence of a thrust-plane between the 



