774 



THE WONDERS OF GEOLOGY. 



Lect. VIII. 



and appear in various detached points, near Dudley. The 

 largest mass constitutes Rowley Hill, a ridge two miles and 

 a half long, and one mile wide, ex- 

 es £ tending from Rowley Regis to the 

 southern suburbs of Dudley. This 

 trap rock, known locally as the 

 Rowley-rag, is a hard, fine-grained, 

 crystalline green-stone, being an 

 admixture of grains of hornblende 

 with small crystals of felspar and 

 quartz. This mineral appears in 

 a slender columnar form in Pearl 

 Quarry, near Timmin's Hill, at 

 *5 Rowley. 



§ But one of the most remark- 

 s' able examples of erupted trap in 

 U this part of England, is that 

 I which has formed the hill called 

 § the Wrekin, near Wellington 

 § in Shropshire, on the north-west 

 flank of the coal-field of Coalbrook 

 Dale ; and which must have taken 

 place after the accumulation of the 

 Silurian strata, as the latter were 

 evidently thrown into inclined 

 positions before the carboniferous 

 were deposited.* At a subsequent 

 period, and long after this con- 

 solidation, the coal-measures were 

 £ -J in their turn pierced and traversed 



* by other intruded masses of ig- 



neous rock, differing in mineral matter, but erupted in 

 contiguous lines of fissure, parallel to that of the Wrekin. 



* The erupted trap forming Barrow Hill (ante, p. 686) is another 

 instructive example of this phenomenon. 



