Page:Popular Science Monthly Volume 10.djvu/280

266 Hydrous silicates act as mineralizers elsewhere than in the greensand. Crinoidal joints from the Silurian limestones of New Brunswick have been saturated by it, filling all the interstices, and small mollusks from Wales have had their interior permeated by it. There is much variation in the composition of these infiltrating silicates. Some from the calcaire grossier, near Paris, approach serpentine. Others carry magnesia. Those from the Lower Silurian of the Upper Mississippi Valley are like glauconite. In the Eozoön, as described above, serpentine, which is a hydrous silicate of magnesia, replaces the supposed sarcodous or animal part of the structure. It has thus corresponded to the glauconite of the present day filling the canals of the supplementary skeleton, the tubuli of the shell, and replacing the softer animal portions. Pyroxene and Loganite also replace the animal matter in the Canadian Laurentian fossils, and in the Eozoön discovered in the supposed Montalban series of Ontario carbonate of lime is the mineralizer. These last-named specimens were not described till 1867; and, as they exhibit the foraminiferal structure without the presence of any form of silicate, they completely establish the genuineness of the fossil. In Bavaria Gümbel states that chondrodite, hornblende, and scapolite, and perhaps other minerals, should be added to the list of silicates petrifying the Eozoön.

The objections that have been made to the organic character of Eozoön relate chiefly to the close resemblances between mineral and organic replacement, or between pseudomorphs and petrifactions. Other resemblances are to dendritic and concretionary structures. Inasmuch as these structures represent the higher efforts of the mineral kingdom in crystallization and the nearest approach to the inorganic world allowed by animal forms, it is not strange that the two extremes should resemble each other sufficiently to deceive practical observers. The canal system may be almost the very picture of certain dendrites. The latter, however, usually occupy a flat surface like moss-agates; whereas the former branch out in every direction, as appears in Fig. 5, projecting upward and downward, as well as sideways.

Organisms are preserved because of the more or less complete substitution of mineral for animal matter. Pieces of coal or wood that have been deposited in clay may be washed out, but the small pores and interstices will be seen to be filled with the matrix. When the burial has been in a solution capable of precipitating solid matter, the wood will be found more or less changed according to the nature of the solution and its capacity for alteration. Some specimens become nearly pure agate in consequence of the gradual substitution, particle by particle, of the organic matter by silica. Fig. 6 shows different stages of petrifaction in coniferous wood: a is a small fragment where the pores have been filled with silica, assuming a somewhat rhomboidal appearance, and the black parts represent the woody substance, still intact; in b the vegetable matter is wanting, having rotted away,