VOLCANIC ROCKS OF THE COAL FIELDS. 303 



yellowish-green streaks. There is a little plagioclase, but the base 

 appears to be chiefly formed of sanidine. Though clearly intrusive, 

 the rock resembles some of the altered volcanic ash in the volcanic 

 series of Borrowdale. 1 



Carboniferous Volcanoes. 



Carboniferous Volcanoes of South Staffordshire. In the South 

 Staffordshire Coalfield there are several masses of basalt, some of 

 which appear to be interstratified with the coal measures. 2 The most 

 important of these outbursts is the columnar basalt, which spreads 

 over an area two miles long by one mile broad in the Rowley Hills, near 

 Dudley. There occur sometimes, just under the basalt, considerable 

 beds of volcanic ash and agglomerate, which seem to show that thu 

 lava was ejected in a true volcanic eruption. The coal, on which 

 it rests, is altered so as to become earthy, and has nearly lost its 

 inflammability. Where this change has taken place, veins sometimes 

 penetrate the coal, which consist of what Professor Jukes termed 

 " white rock," chiefly distinguished from basalt by yielding on ana- 

 lysis a large percentage of carbonic acid and water and a small per- 

 centage of silica, differences which Professor Jukes attributed to the 

 assimilation of a portion of the coal by the basalt vein. There is no 

 trace of the original throat through which the basalt was ejected. 

 Some of the basalts of the Dudley Coalfield appear to be on a different 

 horizon from that of the Rowley Hills, which is 600 feet above the 

 thick coal. 



Among the minor masses is one at Barrow Hill, 10 miles west of 

 Dudley. A smaller columnar mass occurs at Pouk Hill, near Walsall, 

 and is below the thick coal. A fourth mass is seen south of Nether- 

 ton. Professor Jukes also distinguishes, under the name of green- 

 stone, sheet-like masses of basaltic rocks which occur in the lower 

 coal-measures between the Rowley Hills up to Wolverhampton, Bil- 

 ston, and Bentley. On the whole, these eruptions may be referred to 

 the close of the Carboniferous period, because the rocks have been 

 faulted with the coal-measures, so that the more important sheets 

 appear to be contemporaneous. 



Mineral Character of the Carboniferous Dolerites. Dolerites in 

 the South Staffordshire Coalfield are quite typical. In the Rowley 

 Rag the texture is finer than at Pouk Hill. The plagioclase occurs 

 in the usual long prisms, well striated, mixed with pale-brown augite, 

 with green pseudomorphs of olivine, a mineral which at Pouk Hill is 

 usually unaltered. There is always some apatite and a little amor- 

 phous glass. 



At Deep More, N.W. of Walsall, the dolerite is usually much 

 altered, so that the felspar is replaced by chlorite, and chlorite is dis- 

 tributed throughout the rock. Where it meets the coal or shales, it 



1 Geological Magazine, 1878, p. 207. 



2 Jukes' Mem. Geol. Surv. South Staffordshire Coalfield, 1859. 



