408 THE CARBONIFEROUS SYSTEM. 



ever been observed either in recent or fossil plants. The inner set of 

 tap roots vary from two to two and a half inches in length ; the dia- 

 meter at their junction with the base of the trunk being about two 

 inches. The outer set are much smaller, being about one inch in dia- 

 meter at their junction with the horizontal roots, and from one to one 

 and a half inch in length. Very few of either set are strictly conical, 

 although they probably were originally of that shape ; some are 

 squeezed into an elliptical, others into a triangular form ; all have been 

 wrinkled horizontally by the shrinkage due to vertical compression. 

 A thick tuft of broad flattened rootlets radiates from the terminations 

 of the tap roots, and a few indistinct areolae are visible on their sides ; 

 the length of these rootlets does not appear to exceed three or four 

 inches, their width being one-fourth of an inch ; a raised black line 

 runs down the middle of each, similar to that observed in the rootlets 

 of Stigmarias. These short thick tap roots were evidently adapted 

 only to a soft wet soil, such as we may easily conceive was the nature 

 of the first layer of mud deposited upon a bed of peat, which had 

 settled down slightly below the level of the water. 



" We may infer also, from the existence of a layer of shale without 

 fossil plants, immediately over the coal, that the prostrate stems and 

 leaves which occur in such large quantities in the next superincumbent 

 bed, fell from trees growing upon the spot, and were entombed in 

 layers of mud held in suspension in water, which at short intervals 

 inundated the low marshy ground on which they grew ; for had the 

 plants been drifted from a distance, we should find them in the first 

 layer of shale as well as in those higher up. 



" Although the main coal is generally overlaid by shale, yet occa- 

 sionally the shale thins out, and the thick sandstone, which is the next 

 stratum in the ascending order, forms the roof of the coal. In such 

 cases the surface of the peat-bog could not have been level when the 

 shale was deposited upon it, some small patches having been still 

 above water ; and as no upright trees are found in the sandstone roof, 

 it may reasonably be inferred that plants would not vegetate upon 

 the bog itself, a layer of soft mud being necessary in the first instance 

 for germinating the seeds ; but when a plant had once taken root in 

 this mud, its rootlets penetrated downwards into the peat, and furnished 

 an abundant supply of nutriment for the rapid growth of the tree, from 

 the rich mass of decaying vegetable matter beneath." 



The Sydney Coal measures contain not only erect trees, but also nu- 

 merous beds with Naiadites, Cythere, Spirorbis, Fish-scales, etc.; though 

 these do not so frequently overlie coal-seams as at the Joggins. The 

 shales at Sydney are also much more rich than those at the Joggins in 



