180 THE STOEY OF THE EARTH AND MAN. 



a point below. To strengthen the unstable base, the 

 lower joints, especially if the mud had been accumu- 

 lating around the plant, shot out long roots instead of 

 leaves, while secondary stems grew out of the sides 

 at the surface of the soil, and in time there was a stool 

 of Calamites, with tufts of long roots stretching down- 

 wards, like an immense brush, into the mud. \Vhen 

 Calamites thus grew on inundated flats, they would, 

 by causing the water to stagnate, promote the eleva- 

 tion of the surface by new deposits, so that their 

 stems gradually became buried ; but this only favoured 

 their growth, for they continually pushed out new 

 stems, while the old buried ones shot out bundles 

 of roots instead of regular whorls of leaves. 



The Calamites, growing in vast fields along the 

 mai?gins of the Sigillaria forests, must have greatly 

 protected these from the effects of inundations, and 

 by collecting the mud brought down by streams in 

 times of flood, must have done much to prevent the 

 intrusion of earthy deposits among the vegetable 

 matter. Their chief ofiice, therefore, as coal-pro- 

 ducers, seems to have been to form for the Sigillaria 

 forests those reedy fringes •^wtricir/'twhen- inundations 

 took place, would exclude mud, and preveht that 

 mixture of earthy matter in the coal which would have 

 rendered it too impure for use. Quantities of frag- 

 ments of their stems can, However, be detected by the 

 microscope in most coals. 



The modern Mares' Tails have thin-walled hollow 

 stems, and some of the gigantic calamites of the coal 



