Page:Popular Science Monthly Volume 10.djvu/277

Rh this calcareous mixture becomes solidified it is the chalk which abounds in the Cretaceous formation of Europe, and makes up the nummulitic and orbitoidal limestones.

Fig. 2 is a close copy of a small slab of Eozoön, showing what are called the laminated, acervuline, and fragmental portions. The diagonal white line represents the course of a vein of calcite. The dark lines and marks correspond to the sarcode or animal matter of the animal, now consisting of serpentine. Calling the base of the



figure the ocean-floor, there may be said to grow upon it the gelatinous sarcode or dark mass. Upon it grew first the delicate calcareous shell, penetrated by the numerous minute orifices or tubuli, larger pores, and occasionally supports of perpendicular plates. Added to this is the supplemental skeleton without the minute tubuli, but traversed by branching canals. This whole skeleton is represented by the white mass next the dark base, consisting of calcite. These two layers or laminæ constitute the. essential part of the structure, and all the numerous layers above are but repetitions of them. Each lamina may cover several inches square of surface at the bottom of the ocean, and perhaps diminish in size as the organism grew upward. In the sketch the layers are seen to grow thinner toward the top, as if the vital energy became exhausted by the demands made upon it, and the supplemental skeleton first disappears. Finally, we have a mass of rounded chambers irregularly piled up near the top, constituting the "acervuline" structure. We may suppose the growth arrested at this stage, and the sending forth of reproductive germs to found new colonies in the adjacent ground.