We sometimes find examples of volcanoes which, by the action of denuding forces, have had their very foundations exposed to our view. Such examples occur in the Western Isles of Scotland, in the Euganean Hills near Padua in Northern Italy, and in many other parts of the earth's surface. In these cases we are able to trace the ground-plan of the volcanic pile, and to study the materials which have consolidated deep beneath the surface in the very heart of the mountain. In studying these 'basal wrecks' of old volcanoes it is always necessary to bear in mind that the appearance and general characters of a volcanic rock may be completely disguised by chemical changes going on within it. It is through want of attention to this fact that so many mistakes were made by the Wernerian school of geologists who declared that they could find no analogy between the basaltic rocks of the globe and the products of active volcanoes, and were hence led to refer the origin of the former to some kind of 'aqueous precipitation.' Many of the hard and crystalline marbles which are employed as ornamental stones were originally loose masses of shells and corals, as we easily perceive when we examine the polished faces. But these incoherent heaps of organic _débris_ have been converted into a compact and solid rock in consequence of the mass being penetrated by water containing carbonate of lime in solution. Crystals of this substance were deposited in every cavity and interstice of the mass, and thus the accumulation of separate organisms was gradually transformed to a material of great solidity and hardness. [Sidenote: FORMATION OF AMYGDALOIDS.] In precisely the same way loose heaps of scoriæ, lapilli, or pumice may, by the passage through them of water containing various substances in solution, have their vesicles filled with crystals, and thus be converted into the hardest and most solid of rock-masses. Similarly the scoriaceous portions of lava-streams have their vesicles filled with crystalline substances deposited from a state of solution, and are thus converted into a solid mass which may at first sight appear to offer but little resemblance to the vesicular materials of recent lava-streams. To these vesicular rocks which have their cavities filled with crystalline substances geologists apply the name of amygdaloids (_L. amygdalus_, an almond). The cavities in lava-rocks are usually more or less elongated, owing to the movement of the mass while in a still plastic state, and the crystalline materials filling these cavities take the almond-like shape; hence the name.