It is like a leap in history from the days of Pericles and Aspasia to those of King Otho, or from the tents of Runnymede to the Crystal Palace of Sydenham. CHAPTER XII. Trap-pebbles of the boulder--Thickness of the earth's crust unknown--Not of much consequence to the practical geologist--Interior of the earth in a highly heated condition--Proofs of this--Granite and hypogene rocks--Trap-rocks; their identity with lavas and ashes--Scenery of a trappean country--Subdivisions of the trap-rocks--Intrusive traps--Trap-dykes-intrusive sheets--Salisbury Crags--Traps of the neighbourhood of Edinburgh--Amorphous masses--Contemporaneous trap-rocks of two kinds--Contemporaneous melted rocks--Tests for their age and origin--Examples from neighbourhood of Edinburgh--Tufas or volcanic ashes--Their structure and origin--Example of contemporaneous trap-rocks--Mode of interpreting them--Volcanoes of Carboniferous times--Conclusion. In the previous pages, allusion has been made to the trap-pebbles imbedded in the boulder, to the various forms of decay exhibited by granitic and trappean rocks, and to the elevation and depression of the solid crust of the earth. Will the reader bear with me for but a few pages more, while I seek to indicate one or two points of interest in a branch of geology that would abundantly reward a diligent observer? Since the days of Hutton, the investigation of what are called _igneous_ rocks has fallen somewhat into the background, and geologists have given themselves, perhaps too exclusively, to the study of organic remains, so that while the palæontology of the British islands has enjoyed an extensive exploration, but little has been done towards the elucidation of our igneous formations and their accompanying phenomena. Much remains to be accomplished, even in those districts usually regarded as in a manner thread-bare, and he must be but an indifferent observer who cannot add a few gleanings to the general stock of information upon this branch of British geology.