In size, concretions may vary from microscopic dimensions to huge masses, 8, 10, or even more feet in diameter. The variations in shape are also great. They may be spherical, elliptical, discoid, or they may assume more irregular and complex forms (Figs. [371] and [372]). The conditions of growth have much to do with the form. Thus a concretion which starts as a sphere may find growth easier in one plane than another, when it becomes discoid. Two or more concretions sometimes grow together, giving rise to complicated forms. Some of the most complex and fantastic forms are perhaps to be explained in this way. Concretions sometimes take the form of tubes. Some minute tubular concretions were formed about rootlets, but the larger ones appear to owe their form to other influences ([Fig. 374]).
Fig. 375.—Section of a concretion (septarium) the cracks of which have been filled by matter deposited from solution. About half natural size. (Photo. by Church.)
Fig. 376.—Section of a concretion, the cracks in which have been filled by deposition from solution. The filling appears to have wedged the parts of the original concretion apart. The fillings are veins. Some of them show that the vein-material was deposited on both walls. About half natural size. (Photo. by Church.)
One of the most extraordinary features of some concretions of complex form is their symmetry. This may be of various phases; in exceptional cases there is a bilateral symmetry almost as perfect as in the higher types of animals. This is especially true of certain calcareous concretions developed in plastic clays ([Fig. 373]).
Fig. 377.—Septarium from Cretaceous clays near the east base of the Rocky Mountains in Montana. (Photo. by Church.)
Concretions sometimes develop cracks within themselves, and these may then be filled with mineral matter differing in composition or color from that of the original concretions (Figs. [375] and [376]). Concretions the cracks of which have been filled by deposition from solution, are called septaria. They are especially abundant in some of the Cretaceous shales and clays. In not a few cases the filling of the cracks appears to have wedged segments of the original concretion farther and farther apart, until the outer surface of the septarium is made up more largely of vein-matter than of the original concretion ([Fig. 377]). Such concretions are often popularly known as “petrified turtles.”
Concretions of the sort indicated above often develop after the enclosing sedimentary rock was deposited. This is shown, among other things, by the fact that numerous planes of lamination may sometimes be traced through the concretions.