[CHAPTER VI.]
WHAT MAY BE LEARNED FROM EOZOON.
T
The microscope has long been a recognised and valued aid of the geological observer, and is perhaps now in danger of being somewhat overrated by enthusiastic specialists. To the present writer its use is no novelty. When, as a very young geologist, collecting fossil plants in the coal fields of Novia Scotia, I obtained access to the then recently published work of Witham on the "Internal Structure of Fossil Vegetables."[61] Fired by the desire to learn something of the structure of the blocks of fossil wood in my collection, I at once procured a microscope of what would now be considered a very imperfect kind, and proceeded to make attempts to slice and examine my specimens, and was filled with joy when these old blackened stems for the first time revealed to me their wonderful structures. At the same time I extended my studies to every minute form of life that could be obtained from the sea or fresh waters. A few years later (in 1841), when a student in Edinburgh, I made the acquaintance of Mr. Sanderson of that city, who had worked for Nicol and Witham in the preparation of specimens, and learnt the modes which he had employed. Since that time I have been accustomed to subject every rock, earth or fossil which came under my notice to microscopic scrutiny, not as a mere specialist in that mode of observation, or with the parade of methods and details now customary, but with the view of obtaining valuable facts bearing on any investigation I might have in hand. It was this habit which induced my old friend, Sir William Logan, in 1858 and subsequent years to ask my aid in the study of the forms believed or suspected to be organic, which had been discovered in the course of his surveys of the Laurentian rocks. In one respect this was unfortunate. It occupied much time, interfered to some extent with other researches, led to unpleasant controversies. But these evils were more than compensated by the insight which the study gave into the fact of the persistence of organic structures in highly crystalline rocks, and to the modes of ascertaining and profiting, by these obscure remains, while it has guided and stimulated enquiry and thought as to the origin and history of life. These benefits entitle the researches and discussions on Eozoon to be regarded as marking a salient point in the history of geological discovery, and it is to these principally that I would attract attention in the present chapter.
[61] Edinburgh, 1833.
Perhaps nothing excites more scepticism as to the animal nature of Eozoon than the prejudice existing among geologists that no organism can be preserved in rocks so highly crystalline as those of the Laurentian series. I call this a prejudice, because any one who makes the microscopic structure of rocks and fossils a special study, soon learns that fossils and the rocks containing them may undergo the most remarkable and complete mechanical and chemical changes without losing their minute structure, and that limestones, if once fossiliferous, are hardly ever so much altered as to lose all traces of the organisms which they contained, while it is a most common occurrence to find highly crystalline rocks of this kind abounding in fossils preserved as to their minute structure.
Let us, however, look at the precise conditions under which this takes place.
When calcareous fossils of irregular surface and porous or cellular texture, such as Eozoon may have been, or corals were and are, become imbedded in clay, marl, or other soft sediment, they can be washed out and recovered in a condition similar to that of recent specimens, except that their pores or cells, if open, may be filled with the material of the matrix, or if not so open that they can be thus filled, they may be more or less incrusted with mineral deposits introduced by water percolating the mass, or may even be completely filled up in this way. But if such fossils are contained in hard rocks, they usually fail, when these are broken, to show their external surfaces, and, breaking across with the containing rock, they exhibit their internal structure merely,—and this more or less distinctly, according to the manner in which their cells or cavities have been filled with mineral matter. Here the microscope becomes of essential service, especially when the structures are minute. A fragment of fossil wood which to the naked eye is nothing but a dark stone, or a coral which is merely a piece of grey or coloured marble, or a specimen of common crystalline limestone made up originally of coral fragments, presents, when sliced and magnified, the most perfect and beautiful structure. In such cases it will be found that ordinarily the original substance of the fossil remains in a more or less altered state. Wood may be represented by dark lines of coaly matter, or coral by its white or transparent calcareous laminæ; while the material which has been introduced, and which fills the cavities, may so differ in colour, transparency, or crystallization, as to act differently on light, and so reveal the original structure. These fillings are very curious. Sometimes they are mere earthy or muddy matter which has been washed into the cavities. Sometimes they are transparent and crystalline. Often they are stained with oxide of iron or coaly materials. They may consist of carbonate of lime, silica or silicates, sulphate of baryta, oxides of iron, carbonate of iron, iron pyrite, or sulphides of copper or lead, all of which are common materials. They are sometimes so complicated that I have seen even the minute cells of woody structures, each with several bands of differently coloured materials deposited in succession, like the coats of an onyx agate.