THE MICROSCOPE IN GEOLOGICAL INVESTIGATION. 303 



and the lignites of the earliest deposits of these series are, almost uni- 

 versally, either Gymnosperms 1 or Palms. 



696. Descending into the Palaeozoic series, we are presented in the 

 vast coal formations of our own and other countries with an extraordinary 

 proof of the prevalence of a most luxuriant vegetation in a comparatively- 

 early period of the world's history; and the Microscope lends the Geolo- 

 gist essential assistance, not only in determining the nature of much of 

 that vegetation, but also in demonstrating (what had been suspected on 

 other grounds) that Coal itself is nothing else than a mass of decomposed 

 vegetable matter, derived from the decay of an ancient vegetation. The 

 determination of the characters of the Ferns, Sigillarice, Lepidodendra, 

 Calamites, 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 persever- 

 ing search has recently 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 Prof. "W. 

 C. Williamson have shown that they formed a series of connecting links 

 between Cryptogamia and Flowering plants; being obviously allied to 

 Equisetacece, Lycopodiacece, etc., in the character of their fructifications 

 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 wood of which Coal 

 itself consists (these having apparently been reduced to a pulpy state by 

 decay, before the process of consolidation by pressure, aided perhaps by 

 heat, commenced), the traces of structure revealed by the Microscope are 

 of ten sufficient especially in the ordinary ' bituminous' coal not only 

 to determine its vegetable origin, but in some cases to justify the Botan- 

 ist 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 Lyco- 

 podiacece ( 347) of the Carboniferous Flora. The larger the proportion 

 of these granules, the brighter and stronger is the flame with which the 

 coal burns; thus in some blazing cannel-co'dls they abound to such a de- 

 gree as to make up the greater proportion of their substance; whilst in 

 anthracite or i stone-coal,' the want of them is shown by its dull and slow 

 combustion. It is curious that the dispersion of these resinoid granules 

 through the black carbonaceous matter is sometimes so regular, as to give 

 to transparent sections very much the aspect of a section of vegetable 

 cellular tissue, for which they have been mistaken even by experienced 

 microscopists; but this resemblance disappears under a more extended 

 scrutiny, which shows it to be altogether accidental. 



697. In examining the structure of coal, various methods may be fol. 



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

 or pine and fir tribe. 



* See his succession of Memoirs on the Coal-plants, in the recent volumes of 

 the "Philosophical Transactions.*' 



