Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/263

Rh VOLCANIC AcTIoN.] 1100 and a height of 15 feet. So quietly did it advance that Breislak could sail round it in a boat and observe its progress. In passing from a ﬂuid to a solid condition, and thus contracting, lava acquires different structures. Lines of divisional planes or joints traverse it, especially perpendi- cula1' to the upper and under surfaces of the sheet. These lines at various irregular distances cross each other so a.s to divide the rock into rude prisms. Occasionally another series of joints at a right angle to these traverses the mass parallel with its bounding surfaces, and thus the rock acquires a kind of ﬁssile or bedded appearance. The most characteristic structure, however, among volcanic rocks is the prismatic, or, as it is incorrectly termed, “basaltic.” Where this arrangement occurs, as it does so commonly in basalt, the mass is divided into tolerably regular pentagonal, hexagonal, or irregularly polygonal prisms or columns, set close together at a right angle to the main cooling sur- faces. These prisms vary fro111 2 or 3 to 18 or more inches in diameter, and range up to 100 or even 150 feet in length. Considerable discussion has arisen as to the mode in which this columnar structure has been ‘produced. The experiments of Mr Gregory Watt were supposed to explain it by the production of a number of spherical concretions in the cooling mass, and the gradual pressure of those soft balls into hexagonal columns, as the mass contracted in cooling. He melted a mass of basalt, and on allowing it to cool observed that, when a small portion was quickly chilled, it took the form of a kind of slag-like glass, not differing much in appearance. from obsidian ; a larger mass, mare slowly cooled, returned to a stony state. He remarked that during this process small globules make their appear- ance, which increase in size by the successive formation of external concentric coats, like those of an o11ion. And he supposed, as each spheroid must be touched by six others, the whole, if exposed to the same pressure acting in every direction, must be squeezed into a series of hexagons. To account, however, for a long column of basalt, we should have to imagine a pile of balls standing exactly centrically one upon the other, an arrangement which seems hardly possible. The prismatic structure is a species of jointing, due to the contraction of the rock as a whole, and not to the production of any internal peculiarities of texture. The concretionary structure associated with the columnar arises from a common tendency to weather out into nodular forms, and may be observed even where the rock is not columnar. J’:-ismatic forms have been superinduced upon rocks by a high temperature and subsequent cooling, as where coal anl sandstone have been invaded by basalt. They may likewise be observed to arise during the consolidation of a substance, as in the case of starch. In that substance the columnar structure is apt to radiate from certain centres, as may also be seen sometimes in basalt and other igneous rocks. Mr Mallet has recently investigated this subject, and con- cludes that “ all the salient phenomena of the prismatic and jointed structure of basalt can be accounted for upon the admitted laws of cooling, and contraction thereby, of melted rock possessing the known properties of basalt, the essential conditions being a very general homogeneity in the mass cool- mg, and that the cooling shall take place slowly, principally from one or more of its surfaces.”1 In the more perfectly columnar basalts the columns are sometimes articulated, each prism being separable into vertebrae, with a cup and ball socket at each articulation. This peculiarity is traced by Mr Mallet to the contraction 01 each prism in its length and in its diameter, and to the ‘ See an abstract of his paper, Proc. Roy. Soc., January 1875. GEOLOGY 249 consequent production of transverse joints, which, as the resultant of the two contracting strains, are oblique to the sides of the prism, but, as the obliquity lessens towards the centre, assume necessarily, when perfect, a cup-shape, the convex surface pointing in the same direction as that in which the prism has grown. This explanation, however, will hardly account for cases, which are not uncommon, where the convexity points the other way, or where it is sometimes in one direction, sometimes in the other.2 The remarkable spheroids which appear in many weathered igneous rocks besides basalts may probably be due to some of the conditions u11der which the original contractions took place. They are quite untraceable on a fresh fracture of the rock. It is only after some exposure to the weather that they begin to appear, and then they gradually crumble away by the successive formation and disappearance of external weathered crusts or coats, which fall off into sand and clay. Almost all augitic or hornblendic rocks, even granite, exhibit the tendency to decompose into rounded spheroidal blocks. By the outpouring of lava two important kinds of geo- logical change are produced. In the ﬁrst place, the surface of a country is thereby materially changed. Stream-courses, lakes, ravines, valleys, in short all the minor features of a landscape, may be completely overwhelmed under a sheet of lava, 100 feet or more in thickness. The drainage of the district is thus effectually altered, and all the numerous changes which ﬂow from the operations of running water over the land are arrested and made to begin again in new channels. In the second place, considerable alterations may likewise be caused by the effects of the heat and vapours of the lava upon the subjacent or contiguous ground. Instances have been observed in which the lava has actually melted down opposing rocks, or masses of slags, on its own surface. Interesting observations, already referred to, have been made at Torre del Greco under the lava stream which overﬂowed part of that town in 1794. It was found that the window-panes of the houses had been devitriﬁed into a white, translucent, stony substance, that pieces of lime- stone had acquired an open, sandy, granular texture, with- out loss of carbonic acid, and that iron, brass, lead, copper, and silver objects had been greatly altered, some of the metals being actually sublimed. Ve can understand therefore that, retaining its heat for so long a time, a mass of lava may induce many crystalline structures, rearrange- ments, or decompositions in the rocks over which it comes to rest, and proceeds slowly to cool. This is a question of considerable importance in relation to the behaviour of ancient lavas which have been intruded among rocks beneath the surface, and have subsequently been exposed, as will be referred to in the sequel. But, on the other hand, the exceedingly triﬂing change produced even by a massive sheet of lava has often been remarked with astonishment. On the ﬂank of Vesuvius we may see vines and trees still ﬂourishing on little islets of the older land-surface completely surrounded by a ﬂood of lava. Professor Dana has given an instructive account of the descent of a lava-stream fron1 Kilauea in June 1840. Islet-like spaces of forest were left in the midst of the lava, many of the trees being still alive. Where the lava ﬂowed round the trees the stumps were usually consumed, and cylindrical holes or casts remained in the lava, either empty or ﬁlled with charcoal. In many cases the fallen crown of the tree lay near, and so little damaged that the epiphytic plants on it began to grow again. Yet so ﬂuid was the 9 Mr Scrope pointed this out (Geol. J[ag., September 1875), though Mr Mallet (Ibid., November 1875) replied that in such cases the arti- culations must be formed just about the dividing surface between the part of the rock which cooled from above, and that which cooled from below. _ X. — 32