CHAP. VI.] CARBONIFEROUS PERIOD. 95 



of the streams be increased with the increase of the rain- 

 fall, but as the area of the land enlarged and the depth of 

 the sea became less, the sand would be carried farther and 

 farther out over the sea-bottom, and would be arranged in 

 sandbanks and shoals by the sea-currents. So far did this 

 filling-up process go on, that parts of the sea-floor actually 

 became shallow enough for the growth of terrestrial plants, 

 and seams of coal occur in the highest part of the Mill- 

 stone Grit of Lancashire. 



There seems, therefore, good reason for supposing that 

 the Millstone Grit marks a general and rather rapid up- 

 heaval of the whole British region, but it is quite certain 

 that this movement exhausted itself before the commence- 

 ment of the true Coal-measures, for these were undoubtedly 

 formed during a gradual and general subsidence, and it is 

 interesting to note how this subsidence is attended by a 

 reverse change in the texture of the sandstones (see p. 92). 

 As Professor Green remarks, 1 " The gradual subsidence 

 and the ceaseless wear and tear of atmospheric denudation 

 gradually lowered the elevated tracts, so that they were 

 acted on less vigorously by subaerial agencies ; at the 

 same time the rivers, descending by gentler gradients, lost 

 by degrees the power of moving coarse heavy detritus. So, 

 with the lapse of years, the amount of sandy sediment 

 gradually grew less and less, and sandstones formed a 

 gradually decreasing item in the deposits in process of 

 formation." 



In spite of the submergence, however, large tracts of the 

 sea-floor were repeatedly silted up and converted into 

 marshy flats, for the best authorities are agreed that coal- 

 seams are land-growths, though the tracts on which they 

 were accumulated were evidently never raised much above 

 the mean level of the sea. It is not improbable that the 

 coal-seams mark pauses in the progress of the subsidence, 

 1 Coal, its History and Uses," p. 61. 



