(2) The second question requires us to consider the nature and origin of the substances constituting the specimens. Reference has already been made to these in our fifth chapter, but they may be more particularly noticed here in connection with the forms as above described. The calcareous laminæ are usually composed of clear translucent calcite or calcium carbonate, though, as in the case of many later fossils, sometimes replaced by dolomite. It often has the fine granular appearance above referred to, but is nearly always crystalline, and traversed by cleavage planes visible under the microscope.[32] This crystalline structure, as every student of fossils knows, is very common in calcareous fossils of all geological ages. In the thicker laminæ the canals traversing them and branching out in their substance are usually visible under a low power, except when they are filled with calcite similar to that of the laminæ themselves. In this case they can be seen only by very careful management of an oblique and subdued light. When occupied with serpentine, this presents, in a thin slice under transmitted light, a yellowish or brownish colour, and in a specimen decalcified with an acid an opaque white appearance. In some of the larger threads of serpentine, as already stated, this mineral forms a thin outer cylinder with a core of calcite or dolomite within; but this appearance is not common. Here and there, especially in the lower layers, a portion of a tube is filled with the harder mineral pyroxene, which is in some respects similar to serpentine, except that it contains lime as well as magnesia, and is destitute of water as an ingredient The finer tubuli into which the canals ramify are most usually filled with dolomite or magnesian limestone, which has a glossy appearance and higher lustre than the surrounding calcite, and so may be distinguished even in a transparent slice; but these fine dolomite threads are best seen when the surface of a slice is treated with a dilute acid in the cold, in which circumstances the calcite is dissolved, while the dolomite remains as tufts of delicate cylindrical hairs, presenting often a very beautiful appearance under the microscope. Thus, as in many other fossils, what are supposed to have been tubes and tubuli are found not empty, but filled with matter even harder and more resisting than the shell itself. [Footnote 32: Especially when the specimen has been heated or jarred in the process of grinding or polishing.]