that it was deposited at the bottom of the water after the trap had cooled and consolidated, and that consequently the trap must be intermediate in age between the beds on which it rests and those which lie above it; in other words, that it is a _contemporaneous_ rock. Hence, if we know the exact geological position and age of the shales, we know also those of the associated trap, and can thus ascertain that at a certain definite period in the past history of our planet a particular district was the scene of volcanic action. Examples of such contemporaneous traps abound among the carboniferous rocks of central Scotland, especially in Fife and the Lothians (Fig. 35). I may refer again to the vicinity of Edinburgh as affording some excellent illustrations. The eastern part of Arthur's Seat displays a series of basalts and greenstones which can be proved to have been thrown out during the times of the Lower Carboniferous rocks, at a period long anterior to that of the Burdiehouse limestone. The Pentland Hills exhibit on a much greater scale vast sheets of felspathic traps, such as felstones and tufas, traceable in some cases for six or seven miles, which were erupted at a still earlier period.[76] The trap pebbles in our boulder consisted of light yellow and pink felstone, and were derived, I make no doubt, from these Pentland Hill beds, when what forms now the cone of Carnethy, rising well-nigh 1900 feet above the sea, existed as one of a scattered archipelago of islets, or as a sunken rock battered by the waves that scattered its shingle along the floor of what may have been either a shallow sea or a shoaling estuary, where eventually the sand and pebbles hardened into that bed of coarse grey sandstone whence our boulder was derived. [Footnote 76: The geology of Arthur's Seat and Pentland Hills was admirably worked out more than quarter of a century ago by Mr. M'Laren. His work (already referred to) is unfortunately now out of print.]