Nevertheless, it is sometimes possible to make sure that the conglomerate or sandstone, though wholly due to the mechanical destruction of already erupted lavas, was in a general sense contemporaneous with the volcanoes that gave forth these lavas. The detrital formation may be traced perhaps up to the lavas from which it was derived, and may be found to be intercalated in the same sedimentary series with which they are associated. Or it may contain large bombs and slags, such as most probably came either directly from explosions or from the washing down of cinder-cones or other contemporaneously existing volcanic heaps. Examples of such intercalated conglomerates will be given from the Lower Old Red Sandstone of Central Scotland and from the Tertiary volcanic plateaux of the Inner Hebrides. CHAPTER IV Materials erupted at the Surface--Extrusive Series--_continued_. iii. Types of old Volcanoes--1. The Vesuvian Type; 2. The Plateau or Fissure Type; 3. The Puy Type. iv. Determination of the relative Geological Dates of ancient Volcanoes. v. How the Physical Geography associated with ancient Volcanoes is ascertained. Having now taken note of the various materials ejected to the surface from volcanic orifices, we may pass to the consideration of these orifices themselves, with the view of ascertaining under what various conditions volcanic action has taken place in the geological past. We have seen that modern and not long extinct volcanoes may be grouped into three types, and a study of the records of ancient volcanoes shows that the same types may be recognized in the eruptions of former ages. The following chapters will supply many illustrations of each type from the geological history of the British Isles. In dealing with these illustrations, however, we must ever bear in mind the all-powerful influence of denudation. We ought not to expect to meet with the original forms of the volcanoes. Some little reflection and experience may be required before we can realize under what aspect we may hope to recognize ancient and much-denuded volcanoes. It may therefore be of advantage to consider here, in a broad way, which of the original characters are most permanent, and should be looked for as mementoes of ancient volcanoes after long ages of denudation. iii. TYPES OF OLD VOLCANOES The three forms of ancient volcanoes now to be discussed are--1st, the Vesuvian type; 2nd, the Plateau or Fissure type; and 3rd, the Puy type.