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Parr IT, § iii] MACROSCOPIC CHARACTERS OF ROCKS. 89 
have had spherical cavities developed in their mass. When this 
cellular structure is marked by comparatively few and small holes, 
it may be called vesécular ; where the rock consists partly of a roughly 
cellular, and partly of a more compact substance mtermingled, as in 
the slag of an iron furnace, it is said to be slaggy; portions where 
the cells occupy about as much space as the solid part, and vary much 
_ in size and shape, are called scoriaceous, this being the character of 
the rough clinker-like scoriz of a recent lava stream; when the 
cells are so much more numerous than the solid part, that the stone 
would almost or quite float on water, the structure is called pumi- 
ceous, the term pumice being the name given to the froth-like part 
of obsidian. As the cellular structure is necessarily developed while 
the rock is still liquid, or at. least viscid, and as while in this con- 
dition the mass is often still moving away from its point of emission, 
the cells are not infrequently elongated in the direction of movement. 
Subsequently water infiltrating through the rock, deposits various 
mineral substances (calcite, quartz, calcedony, zeolites, &c.) from 
solution, so that the flattened and elongated almond-shaped cells are 
eventually filled up. A rock which has undergone this change is said to 
be amygdaloidal, and the almond-like kernels are known as amygdules. 
Concretionary, containing or consisting of mineral matter 
which has been collected, either from the surrounding rock or from 
without, round some centre, so as to form a nodule or irregularly 
shaped lump. This aggregation of material is of frequent occurrence 
among water-formed rocks, where it may be often observed to have 
taken place round some organic centre, such as leaves, cones, shells, 
fish remains, or other relics of plants or animals, (Book IV. Part 1.) 
Among the most frequent minerals found in concretionary forms as 
constituents of rocks are calcite, siderite, pyrite, marcasite, and 
various forms of silica. In a true concretion the material at the 
centre has been deposited first, and has increased by additions from 
without, either during the formation of the enclosing rock, or by 
subsequent concentration and aggregation. Where, on the other 
hand, cavities and fissures have been filled up by the deposition of 
materials on their walls, and gradual growth inward, the result is ~ 
known as a secretion. Amygdules and the successive coatings 
of mineral veins are examples of the latter process. | 
Spherulitic, composed of, or containing small globules or 
_ spherules which may be colloid and isotropic or more or less dis- 
tinctly crystalline, particularly with an internal fibrous divergent 
structure. This structure occurs in vitreous rocks, where it is one of 
the stages of devitrification in obsidian, pitchstone, We. (see p. 141). 
Perlitic, having the structure of the rock termed perlite, 
which is distinguished by being traversed by minute rectilinear 
fissures, between which the substance of the mass has assumed a 
finely globular character, not unlike the spheroidal structure seen 
in weathered basalt (Fig. 22). 
Granitoid, thoroughly crystalline, and consisting of crystals 
