236. Though the argument here advanced for the igneous origin of the rock may be admitted as conclusive, the introduction of calcareous spar into it by infiltration must still be questioned. Lava, except in a state of decay or decomposition, is not readily penetrated by water; and, if it were, the filling of cavities with spar, by means of the water percolating through them, would still be subject to many difficulties, (§12.). Besides, whinstone rocks are frequently found so full of calcareous spar, or of zeolite, that they would become porous to such a degree, if the cavities filled with these latter substances were all empty, that they could hardly sustain their own weight, and much less that of the great masses of rock incumbent on them. In such cases, it is certain, that the crystallized substances were part of the original composition of the rock. The truth is, that the infiltration of the water is a mere gratuitous assumption, introduced for the purpose of explaining the existence of carbonated lime in a stone which had endured the action of intense heat: and this assumption ought of course to be rejected, if the phenomenon can be explained by a theory, that is in other respects conformable to nature. The spar, then, may be considered as a proof, that the rocks in question are to be numbered with those unerupted lavas which have flowed deep in the bowels of the earth, and under a great compressing force. This is the more probable, that the Euganean Hills, like some whinstone hills in our own country, have, in certain places, a covering of slaty and calcareous strata incumbent on them, even at their summits,[121] so that the torrent of melted stone, of which they are admitted to consist, cannot have flowed from the mouth of a volcano. I do not mean to say, that there are among these hills no vestiges of volcanic explosion. I am very far from having _data_ sufficient for drawing this conclusion; but I believe it may be safely affirmed, that the bulk of them is no more composed of volcanic lava, than the basaltes of Staffa, or of the Giant's Causeway. [Footnote 121: Phil. Trans. 1775, p. 34.]