It is also found that all rocks are divided into huge blocks by seams not parallel with the cleavage, and too regular to be considered as fractures. These seams bear an analogy to the secondary faces of crystals, which are never parallel to the cleavage. 2. _Concretionary Formations._--There exist in many rocks concretions which differ from the mass of the rocks. In most of the tertiary clays there are small concretionary nodules, which contain more calcareous matter than the mass of clay around them. In the coal formation, the nodular iron ore consists of concretionary masses. In the chalk formation, nodules of flint abound, and generally in layers. In many of these cases, particularly in the clays and coal, the nodules have an organic nucleus, and, although concretionary, they retain the marks of stratification of the adjacent rocks. Hence they could not have been deposited in the form of nodules. There must therefore have been in the rock, though in the solid state, such motion among the molecules that particles of a particular mineral have separated from the mass and rearranged themselves in concretionary layers, yet so gradually as not to disturb the lines of original stratification. [Illustration: Fig. 70.] There are other instances, similar to the last in all respects, except that the segregated portion does not take the concretionary form. When gypsum is distributed in small proportion through a formation, there seems very little reason to doubt but that it is, by a molecular action, segregated from the strata in lenticular masses, as at a (Fig. 70). Many of the limestone strata contain irregular aggregations of quartz. It is presumed that the siliceous and calcareous matter was deposited together as sediment, and that the aggregation has resulted from a movement among the particles similar to that by which the concretionary structure is produced. The columnar structure of basalt seems to have resulted from a peculiar molecular action, at first resembling a concretionary arrangement, while the mass was cooling from a state of fusion. In experimenting to ascertain the cause of this structure, Mr. Watt fused in a furnace seven hundred pounds of basalt. When cooled, he found that "numerous spheroids had been formed, and that when two of them came in contact, they did not penetrate each other, but were mutually compressed and separated by a well-defined plane, invested with a rusty coating. When several met, they formed prisms." (Fig. 71.) [Illustration: Fig. 71.] 3. _Mineral Veins._--The phenomena of veins are such that they cannot all be referred to the same cause. In some, the vein-stuff has been protruded as a dike, differing from ordinary dikes only in the accidental circumstance that it contains a metal or a metallic ore.