1084 THE MICEOSCOPE IN GEOLOGICAL INVESTIGATION 



the earliest deposits of these series are, almost universally, either 

 gymnosperms l or palms. 



Descending into the palaeozoic series, we are presented in the 

 vast coal formations of our own and other countries with an extra- 

 ordinary proof of the prevalence of a most luxuriant vegetation in a 

 comparatively early period of the world's history. The determina- 

 tion of the characters of the Ferns, Sigillarice, Lepidodendra, Cala- 

 mites, and other kinds of vegetation whose forms are preserved in 

 the shales or sandstones that are interposed between the strata of 

 coal, has been hitherto chiefly based on their external characters ; 

 since it is seldom that these specimens present any such traces 

 of minute internal structure as can be subjected to microscopic 

 elucidation. But persevering search has brought to light numerous 

 examples of coal-plants whose internal structure is sufficiently well 

 preserved to allow of its being studied microscopically ; and the 

 careful researches of Professor W. C. Williamson have shown that 

 they formed a series of connecting links between Cryptogainia 

 and flowering plants, being obviously allied to Equiselacece, Lyco- 

 podiacece, &c., in the character of their fructification, whilst their 

 stem-structure foreshadowed both the ' endogenous ' and ' exogenous ' 

 types of the latter. 2 Notwithstanding the general absence of any 

 definite form in the masses of decomposed vegetable matter of which 

 coal itself consists, the traces of structure revealed by the microscope 

 are often sufficient especially in the ordinary ' bituminous ' coal 

 not only to determine its vegetable origin, but in some cases to 

 justify the botanist in assigning the character of the vegetation 

 from which it must have been derived ; and even where the stems 

 and leaves are represented by nothing else than a structureless mass 

 of black carbonaceous matter, there are found diffused through this 

 a multitude of minute resinoid yellowish-brown granules, which are 

 sometimes aggregated in clusters and inclosed in sacculi ; and these 

 may now be pretty certainly affirmed to represent the spores, while 

 the sacculi represent the sporangia, of gigantic Lycopodiacece of the 

 Carboniferous flora. 3 



Lime-secreting alga? are now known to have often played an 

 important part in the formation of calcareous rocks. Those 

 organisms called coccoliths and rhabdoliths, which though so 

 minute are important constituents in chalk and some other lime- 

 stones, are referred to these plants (? to the class Floridece), and a 

 tiny tubular organism named Girvanella which occurs in various 

 palaeozoic and later limestones is now generally regarded as an 

 alga. According to Mr. E. Wethered 4 it plays an important part 

 in the formation of pisolitic and oolitic grains. Moreover 

 calcareous algae, such as Lithothamnion, are sometimes important 

 constituents in Tertiary limestones, as for instance in the Leitha- 



1 Under this head are included the Cycadece, along with the ordinary Conifera, 

 or pine and fir tribe. 



2 See his memoirs on the coal-plants published in the volumes of the Phil. Trans., 

 which are now being continued by Dr. D. H. Scott. 



3 For notes upon methods to be employed in making preparations of coal, see 

 Rutley, Study of Bocks, 1884, p. 71. 



4 Quart. Journ. Geol. Soc. xlvi. (1890), p. 270, xlviii. p. 377, xlix. p. 236. 



