302 THE MICROSCOPE AND ITS REVELATIONS. 



CHAPTER XXL 



APPLICATION OF THE MICROSCOPE TO GEOLOGICAL INVESTIGATION. 



694. THE utility of the Microscope is by no means limited to the deter- 

 mination of the structure and actions of the Organized beings at present 

 living on the surface of the Earth ; for a vast amount of information is 

 afforded by its means to the Geological inquirer, not only with regard to 

 the minute characters of the many Vegetable and Animal remains that 

 are entombed in the successive strata of which its crust is composed, but 

 also with regard to the essential nature and composition of many of those 

 strata themselves. We cannot have a better example of its value in both 

 these respects, than that which is afforded by the results of Microscopic 

 examination of lignite or fossilized wood, and of ordinary coal, which 

 we now assuredly know to be a product of the decay of wood. 



695. Specimens of fossilized tvood, in a state of more or less complete 

 preservation, are found in numerous strata of very different ages, more 

 frequently, of course, in those whose materials were directly furnished by 

 the dry land, and were deposited in its immediate proximity, than in 

 those which were formed by the deposition of sediments at the bottom of 

 a deep ocean. Generally speaking, it is only when the wood is found to 

 have been penetrated by silex, that its organic structure is well preserved; 

 but instances occur every now and then, in which penetration by carbon- 

 ate of lime has proved equally favorable. In either case, transparent sec- 

 tions are needed for the full display of the organization; but such sections, 

 though made with great facility when lime is the fossilizing material, 

 require much labor and skill when silex has to be dealt with. Occasion- 

 ally, however, it has happened that the infiltration has filled the cavities 

 of the cells and vessels, without consolidating their walls ; and as the latter 

 have undergone decay without being replaced by any cementing material, 

 the lignite, thus composed of the internal ' casts ' of the woody tissues, is 

 very friable, its fibres separating from each other like those of asbestos ; 

 and laminae split asunder with a knife, or isolated fibres separated by 

 rubbing-down between the fingers, exhibit the characters of the woody 

 structure extremely well, when mounted in Canada balsam. Generally 

 speaking, the lignites of the Tertiary strata present a tolerably close 

 resemblance to the woods of the existing period: thus the ordinary struc- 

 ture of dicotyledonous and monocotyledonous stems may be discovered in 

 such lignites in the utmost perfection ; and the peculiar modification pre- 

 sented by coniferous wood is also most distinctly exhibited (Fig. 259). 

 As we go backj however, through the strata of the Secondai / period, we 

 more and more rarely meet with the ordinary dicotyledonoas structure*.. 



