154 ON THE ORIGIN OF COAL. 



during the elevation or period of rest of the 

 former. The whole of its materials, and the few 

 organic remains found in its fossils, as well as 

 the sand of which it is composed, show that it 

 was not deposited in a deep ocean, as these are 

 seldom met with at great depths in our present 

 seas. The grains of sand also indicate consider- 

 able currents, which we should not generally 

 expect to find in very deep water. The thickness 

 of this formation is very great, reaching, according 

 to Murchison, (p. 184 of his Silurian System,) 

 to nine or ten thousand feet ; a depth of sea which 

 the composition of the rocks and the organic 

 remains found in them, seems to render it next 

 to impossible, but that subsidences of its bottom 

 frequently took place during its formation. From 

 the conformability of the rocks in some position's, 

 it is now generally admitted that the old red 

 sandstone in some places passes upwards into 

 the mountain limestone, as at Stockpole Cliffs, 

 (p. 383 Murchison's Silurian System,) and many 

 other localities. However, in the north east of 

 Wales, this transition is not to be observed, but 

 the mountain limestone reposes on unconformable 

 Silurian rocks. 



The mountain limestone, or, as it is now 



