628 MICROSCOPIC GEOLOGY. 



balsam. Generally speaking, the lignites of the Tertiary strata 

 present a tolerably close resemblance to the wood of the existing 

 period ; thus the ordinary structure of Dicotyledonous and Mono- 

 cotyledonous stems, may be discovered in such lignites in the 

 utmost perfection; and the peculiar modification presented by 

 Coniferous wood, is also most distinctly exhibited (Fig. 171). 

 As we descend, however, through the strata of the Secondary 

 period, we more and more rarely meet with the ordinary dicoty- 

 ledonous structure; and the lignites of the earliest deposits of 

 these series are, almost universally, either gymnosperms or 

 palms. 1 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 geologist essential assistance, not 

 only in determining the nature of much of that vegetation, but 

 also in demonstrating, what has been suspected on other grounds, 

 that Coal itself is nothing else than a mass of decomposed vege- 

 table matter, chiefly derived from the decay of Coniferous wood. 

 The determination of the characters of the Ferns, Sigillarice, 

 Lepidodendra, Calamites, and other kinds of vegetation whose 

 forms are preserved in the shales and sandstones that are inter- 

 posed between the strata of coal, must be chiefly based on their 

 external characters ; since it is very seldom that any of the speci- 

 mens present any such traces of minute internal structure, as 

 can be subjected to microscopic elucidation. But notwithstand- 

 ing the general absence of any definite form in the masses of de- 

 composed wood of which Coal itself consists (these having ap- 

 parently 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 sufficient not only to determine its vegetable origin, but, in 

 some cases, to justify the Botanist in assigning the characters of 

 the vegetation from which it must have been derived. Different 

 specimens of Coal exhibit these structural characters in very 

 different degrees of distinctness; but they uniformly indicate, 

 with a clearness proportionate to their distinctness, that such 

 vegetation must have been Coniferous in its nature, and that it 

 probably approximated most nearly to that group of existing 

 Conifers, to which the Araucarioe belong. These inferences 

 are based upon the fact, that the woody structure consists of 

 woody fibres without interposed vessels ; upon the presence of 

 glandular dots on the woody fibres ; and upon the peculiar 

 arrangement of these dots in two or more rows, alternating one 

 with another ( 231, 238). 



443. In examining he structure of Coal, various methods may 

 be followed. Of those kinds which have sufficient tenacity, thin 

 sections may be made ; but the opacity of the substance requires 



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

 and fir tribe. 



