CARBONIFEROUS. 1 05 



in which they fell, before they became matted 

 and compressed into a solid mass. 



In some ll growth has been com- 



pared to the American swamps and cane-brakes, 

 where the girdle of surrounding vegetation filters 

 the muddy waters, so that only clear water 

 reaches the vegetable matter in the enclosure. 



There is no doubt that carboniferous land SUr- 



itly undergoing depression oi 



el, like the Deltas "t" rivers Mich as the Po, 



and the Mississippi, in many of which a suCi 



sion of land surfaces has been found one below 

 ther, indicating aulation of sediments 



which art' similar to the coal shales and sand- 



semble them, in the intercalation of 

 ve£< growths between layers of mud. These 



depressions during the growth of the coal often 



appear to have been partial and local. 



In the Dudley coal field tin- to-yard scam is 

 found, which is the thickest coal bed in England. 



When it is traced to the north, it subdivides into 

 nine Seams of h having its own bed of 



under clay, on whii lis 5SH eStS grew. At 



Bssington the nine beds preserve the thickness 



the one bed at Dudley, though they have become 

 irated from each other, by wedge shaped 

 ers <»f sandstones and shales, which have an 

 te thick: ; jo feet. 



The Dudley country is also interesting among 

 English coal-fields on account of the volcanic 

 eruption, which appears to have taken place dur- 

 ing the carboniferous period; for the basalt at 

 Rowley Regis, which was ejected through the 

 coal, is in some places in the condition of cinder 

 and ash ; and this appears to prove that it was 

 eiec'ed it or near the surface. Volcanic out- 



