as widely as in any other stratified rock, being sometimes land-plants, sometimes sea-shells. Such are some of the more obvious characters of the volcanic ashes or tufas, as developed among the carboniferous rocks of central Scotland. Their great varieties of composition and general aspect render them a somewhat difficult set of rocks to master, but when fairly and fully understood they soon prove themselves to be by far the most interesting section of the traps, for one needs seldom to hesitate a moment as to their origin or date, while their fossil contents impart to them an interest all their own. By comparing such rocks with the consolidated ash or fine dust and _lapilli_ of a modern volcano, a remarkable resemblance of external characters is found to subsist; and this likeness holds sufficiently close, when pursued into details, to show that the ancient and the modern rocks have resulted from the same source, that, namely, of volcanic eruption. The ash of active burning mountains falls down their sides loosely and incoherently, every successive shower of dust or scoriæ settling without much regularity on those that have gone before. The ash of the old carboniferous eruptions, however, was showered for the most part over the sea or across wide shoaling estuaries, at least it is only such portions of it as fell there that have come down to our day. Settling down among the mud and sand at the bottom, the volcanic matter accumulated in wide horizontal beds, every marked inequality being smoothed down by the currents until a series of regularly stratified layers came to be formed, entombing any organisms that might find their way to the bottom or be lying there at the time. The ash of terrestrial volcanoes has no marked stratification because thrown out in open air, while that of the carboniferous rocks of central Scotland is distinctly bedded from having been deposited under water.