Page:Proceedings of the Royal Society of London Vol 1.djvu/174

Rh of its formation by water must be allowed to be at least of equal weight. In fact, while the frequent instances of petrifactions found in basalt support the aqueous hypothesis, the equally numerous indi< cations that the heat emanating from it has manifestly changed beds of coal into coke and indurated strata of stony substances, strongly argue in favour of the igneous assumption. -

The above sketch of the author’s observations and reasonings, im- perfect as it is, may however suﬂice for rightly comprehending the interesting part of the paper which relates to the wonderful regula< rity of the prismatic conﬁguration of basaltic columns, and also for their articulations. If we suppose that a mass of ﬂuid basalt has ﬁlled a valley to an indeﬁnite depth and extent. the process of ar- rangement in its particles must be deduced from the removal of its heat or moisture, according as its solution is igneous or aqueous. This can only be done by the action of the atmosphere on its upper surface, and by the ground on which it reposes absorbing the heat or moisture from its under surface.

From the variations of the atmosphere, its action must be irregu- lar; and from the perpetual change of the parts in contact with the heated or moist surface, its operations will always be nearly as active as at ﬁrst, allowance being made for its variations : but the absorp- tion of the ground will be regular, and regularly diminishing in ac- tivity, in proportion as the parts near the mass approach nearer to the same temperature or same moisture with the mass abOVe; and thus absorption can only be carried on by the transmission of heat or moisture from the mass to the solid rocks below.

From these considerations, it seems evident that the arrangement of the part of the basaltic mass near the ground will be begun with more energy than it can be continued, and that the results will be more slow and regular, and that induced by the action of the atmo- sphere. After the ﬁrst stage in the process of arrangement has been performed, and a stratum of the jaspideous substance is extended over the surface of the ground, there seems no reason to doubt that a number of radiated spheroids will be generated in it, having pro- bably all their centres nearly at the same distance from the ground; and as the arranging power undergoes a gradual diminution of energy, it is not likely that two rows of them in height should be formed at once. In a word, it seems most probable, that in the arrangement of a mass of basalt, a single layer of radiated spheroids will be form- ed, reposing on the ground which supports the mass.

How these radiated spheroids, by coming in close contact, will compress each other, and form polyhedral and, generally, hexagonal prisms, will be understood from what we have said above of that operation in the experiment. If these prisms are resisted below, and there is no opposing cause above them, it is clear that they will ex- tend their dimensions upwards into the undisturbed central mass of the ﬂuid, till their structure is deranged by the action of the atmo- sphere on the upper surface of the basalt. According to this arrange ment. the same cause that determines the concentric fractures of the