Some Salient Points in the Science of the Earth

by Sir John William Dawson

Fig. 15.—Walls of Eozoon penetrated with Canals. The unshaded portions filled with Calcite. (After Carpenter.)

In the case of recent and fossil Foraminifers, these very frequently have their chambers filled solid with calcareous matter, and as Dr. Carpenter well remarks, even well-preserved Tertiary Nummulites in this state often fail greatly in showing their structures, though in the same condition they occasionally show these in great perfection. Among the finest I have seen are specimens from the Mount of Olives, and Dr. Carpenter mentions as equally good those of the London clay at Bracklesham. But in no condition do modern Foraminifera, or those of the Tertiary and Mesozoic rocks appear in greater perfection than when filled with the hydrous silicate of iron and potash called glauconite or green earth, a substance now forming in some parts of the ocean, and which gives, by the abundance of its little bottle-green concretions the name of "greensand" to formations of the Cretaceous age both in Europe and America. In some beds of greensand every grain seems, to have been moulded into the interior of a microscopic shell, and has retained its form after the frail envelope has been removed. In some cases the glauconite has not only filled the chambers but has penetrated the fine tubulation, and when the shell is removed, either naturally or by the action of an acid, the silicious fillings of the interior of the tubes project in minute needles or bundles of threads of marvellous delicacy from the surface of the cast. It is in the warmer seas, and especially in the bed of the Egean and of the Gulf Stream, that such specimens are now most usually found.[63] If we ask why this mineral glauconite should be associated with foraminiferal shells, the answer is that they are both products of one kind of locality. The same sea bottoms in which Foraminifera most abound are also those in which the chemical conditions for the formation of glauconite exist. Hence, no doubt, the association of this mineral with the great foraminiferal formation of the chalk. It is indeed by no means unlikely that the selection by these creatures of the pure carbonate of lime from the sea water or its minute plants, may be the means of setting free the silica, iron, and potash, in a state suitable for their combination. Similar silicates are found associated with marine limestones, as far back as the Cambro-Silurian age; and Dr. Sterry Hunt, than whom no one can be a better authority on chemical geology, has argued on chemical grounds that the occurrence of serpentine with the remains of Eozoon is an association of the same character.

[63] Beautiful specimens of Nummulites preserved in this way, from the Eocene of Kumpfen in Bavaria, have been communicated to me through the kindness of Dr. Otto Hahn.

However this may be, the infiltration of the pores of Eozoon with serpentine and other silicates has evidently been one main means of its preservation. When so infiltrated no metamorphism short of the complete fusion of the containing rock could obliterate the minutest points of structure; and that such fusion has not occurred, the preservation in the Laurentian rocks of the most delicate lamination of the beds shows conclusively; while, as already stated, it can be shown that the alteration which has occurred might have taken place at a temperature far short of that necessary to fuse limestone. Thus has it happened that these most ancient fossils have been handed down to our time in a state of preservation comparable, as Dr. Carpenter states, to that of the best preserved fossil Foraminifera from the more recent formations that have come under his observation in the course of all his long experience.

Let us now look more minutely at the nature of the typical specimens of Eozoon as originally observed and described, and then turn to those preserved in other ways, or more or less destroyed or defaced. Taking a polished specimen from Petite Nation, we find the shell represented by white limestone, and the chambers by light green serpentine. By acting on the surface with a dilute acid we etch out the calcareous part, leaving a cast in serpentine of the cavities originally occupied by the soft animal substance, and when this is done in polished slices, these may be made to print their own characters on paper, as has actually been done in the plate prefixed, which is an electrotype from an etched specimen, and shows both the laminated and acervuline parts of the fossil. If the process of decalcification has been carefully executed, we find in the excavated spaces delicate ramifying processes of opaque serpentine or transparent dolomite, which were originally imbedded in the calcareous substance, and which are often of extreme fineness and complexity.[64] (Figs. [18], [19].) These are casts of the canals which traversed the shell when still inhabited by the animal, and have subsequently been filled with mineral matter. In evidence of this we sometimes find in a single canal an outer tubular layer of serpentine and an inner filling of dolomite, just as vessels of fossil plants are sometimes filled with successive coats of different materials. In some well preserved specimens we find the original cell wall represented by a delicate white film, which under the microscope shows minute needle-like parallel processes representing its still finer tubuli. It is evident that to have filled these tubuli, the serpentine must have been introduced in a state of actual solution, and must have carried with it no foreign impurities. Consequently we find that in the chambers themselves the serpentine is pure; and if we examine it under polarized light, we see that it presents a singularly curdled or irregularly laminated appearance, as if it had an imperfectly crystalline structure, and had been deposited in irregular laminæ, beginning at the sides of the chambers, and filling them toward the middle, and had afterward been cracked by shrinkage, and the cracks filled with a second deposit of serpentine.[65] Now, serpentine is a hydrous silicate of magnesia, and all that we need to suppose is that in the waters of the Laurentian sea magnesia was present instead of iron, alumina or potash, and we can understand that the Laurentian fossil has been petrified by infiltration with serpentine, as more modern Foraminifera have been with glauconite, which, though it does not contain magnesia, often has a considerable percentage of alumina. Further, in specimens of Eozoon from Burgess, the filling mineral is loganite, a compound of silica, alumina, magnesia and iron with water, while in other specimens the filling mineral is pyroxene. In like manner, in certain Silurian limestones from New Brunswick and Wales, in which the delicate microscopic pores of the skeletons of stalked starfishes or crinoids have been filled with mineral deposits, so that when decalcified these are most beautifully represented by their casts, Dr. Hunt has proved the filling mineral to be[66] intermediate between serpentine and glauconite. We have, therefore, ample warrant for adhering to his conclusion that the Laurentian serpentine was deposited under conditions similar to those of the modern greensand. Indeed, independently of Eozoon, it is impossible that any geologist who has studied the manner in which this mineral is associated with the Laurentian limestones could believe it to have been formed in any other way. Nor need we be astonished at the fineness of the infiltration by which these minute tubes, perhaps 1/10000 of an inch in diameter, are filled with mineral matter. The micro-geologist well knows how, in more modern deposits, the finest pores of fossils are filled, and that mineral matter in solution can penetrate the smallest openings that the microscope can detect. Wherever the fluids of the living body can penetrate, there also mineral substances can be carried, and this natural injection, effected under great pressure and with the advantage of ample time, can surpass any of the feats of the anatomical manipulator. [Fig. 16] represents a microscopic joint of a Crinoid from the Upper Silurian of New Brunswick, injected with the hydrous silicate already referred to, and [Fig. 17] shows a microscopic chambered or spiral shell, from a Welsh Silurian limestone, with its cavities filled with a similar substance.

[64] Very fine specimens can be produced by polishing thin slices, and then etching them slightly with a very weak acid. (Plate prefixed.)

[65] The same structures may be well seen in thin slices polished, to be viewed as transparent objects. I may, however, explain that if these are made roughly, and heated in the process, they may often show only mineral structures and cleavage planes, whereas, if polished with great care and slowly, and afterwards cleaned with an acid, they may show the canals in great perfection.

[66] Silicate of alumina, iron, magnesia, and potash.