THE CARBONIFEROUS AGE. Ill 



vening cement, is the cause of the colour. In finer 

 sandstones and red clays the same condition exists, 

 though less distinctly perceptible. Conrrequently, if 

 red sands and clays are long abraded or scoured in 

 water, or are subjected to any chemical agent capable 

 of dissolving the iron, they cease to be red, and re- 

 sume their natural grey or white colour. Now in 

 nature, in addition to mechanical abrasion, there is a 

 chemical cause most potent in bleaching red rocks, 

 namely, the presence of vegetable or animal matter 

 in a state of decay. Without entering into chemical 

 details, we may content ourselves with the fact that 

 organic matter decaying in contact with peroxide of 

 iron tends to take oxygen from it, and then to dis- 

 solve it in the state of protoxide, while the oxygen 

 set free aids the decay. Carrying this fact with us, 

 we may next affirm that iron is so plentiful in the 

 crust of the earth that nearly all sands and clays 

 when first produced from the weathering of rocks 

 are stained with it, and that when this weathering 

 takes place in the air, the iron is always in the state 

 of peroxide. More especially does this apply to the 

 greater number of igneous or volcanic rocks, which 

 nearly always weather brown or red. Now premising 

 that the original condition of sediment is that of 

 being reddened with iron, and that it may lose this 

 by abrasion, or by the action of organic matter, it 

 follows that when sand has been produced by decay 

 of rocks in the air, and when it is rapidly washed 

 into the sea and deposited there, red beds will result, 



