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They probably fell in successive generations from natural decay; and making every allowance for other materials, we may safely assert that every foot of thickness of pure bituminous coal implies the quiet growth and fall of at least fifty generations of Sigillarioe, and therefore an undisturbed condition of forest growth enduring through many centuries.
The climate of the period, in the northern temperate zone, was of such a character that the true conifers show rings of growth, not larger, nor much less distinct, than those of many of their modern congeners. The Sigillarioe and Calamites were not, as often supposed, composed wholly, or even principally, of lax and soft tissues, or necessarily short-lived.
Though the roots of the Sigillaria bear more resemblance to the rhizomes of certain aquatic plants; yet, structurally, they are absolutely identical with the roots of Cycads, which the stems also resemble. Further, the Sigillarioe grew on the same soils which supported Conifers, Lepidodendra, Cordaites, and Ferns-plants which could not have grown in water.
In the case of the Sigillarioe, the variations in the leaf-scars in different parts of the trunk, the intercalation of new ridges at the surface representing that of new woody wedges in the axis, the transverse marks left by the stages of upward growth, all indicate that several years must have been required for the growth of stems of moderate size.
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