XIV.] LIVING MATTER AND ITS EFFECTS. 235 



above the seam, is commonly a shale, which when split into 

 layers is very frequently found to inclose impressions of 

 plants. Perhaps the commonest of these remains are the 

 graceful leaves oxfro?!ds'^ of ferns, which are often strikingly 

 similar to those living at the present day. Although, in 

 these islands, ferns never attain to the size of trees, yet, in 

 countries where the climate is very warm and moist, as in 

 New Zealand, they form trees fifty or sixty feet in height. 



Such tree-ferns lived also in this country, at the time of the 

 formation of the shales which are found in association with 

 the coal. 



In addition to the impressions of plants found in the shales 

 above the coal seams, vegetable remains are also met with 

 in rocks beneath the coal, forming what is called the " floor." 

 It was pointed out many years ago by the late Sir W. Logan, 



^ A frond differs from an ordinary leaf in usually bearing fructifica- 

 tion. The ferns have no flowers, and the fruit is generally developed 

 on the frond. 



