COAL: ITS STRUCTURE AND FORMATION. 367 



of a gelatinous alga, to which the name Pila bibractensis has 

 been assigned. The thallus is divided up into a number of 

 small thick-walled cells, and in some of the cell cavities- 

 thanks to their wonderful preservation in silica— protoplasm 

 and nuclei have been recognised. We may be permitted 

 to express grave doubts as to the possibility of such well-nigh 

 incredible statements. In addition to the numberless ex- 

 amples of Pila bibractensis, pollen grains are not uncommon. 

 The algal thallus and pollen grains are embedded in a brown 

 ground mass which constitutes a kind of amorphous precipi- 

 tate charged with vegetable fragments. The conclusion 

 arrived at is that the Autun boghead is the product of an 

 immense accumulation of a single species of gelatinous alga, 

 with grains of pollen and other plant structures, in a matrix 

 of ulmic substances. The brown colour suggests the coffee- 

 coloured waters of some tropical rivers, and the algse may 

 be regarded as analogous to the Jieurs d'eau of fresh-water 

 lakes. In the calm, brown waters of a Permo-Carboniferous 

 lake, ulmic materials were precipitated by the action of car- 

 bonated waters ; and at certain seasons of the year the 

 surface of the water was covered with a mass of microscopic 

 algae, and these, with showers of pollen from neighbouring- 

 forests, accumulated as a pulpy mass of ulmic products on 

 the flora of the lake, and so gave rise to a deposit of bog- 

 head. A similar structure is recognised by these authors in 

 an Australian boghead, and in the torbanite of Scotland. 

 In the Australian boghead Reinschia australis, another 

 gelatinous alga, has played the most important rdle in build- 

 ing up the carbonaceous material ; and in the Scotch rock 

 another species of Pila is the characteristic constituent. In 

 addition to the algal species, an aquatic Myxomycete is re- 

 corded from the Autun boghead, described under the name 

 of Bretonia Haidingeri ; the same orenus has also been 

 found in the Scotch beds. 



It should be noted that the Autun bogheads may occur 

 mixed with ordinary coal, and that coal is sometimes found 

 in the form of lenticular patches in a bed of boghead. The 

 results arrived at by Bertrand and Renault compel us to 

 adopt a somewhat sceptical attitude in attempting to form an 



