In themselves, accumulations of volcanic materials do not furnish any exact or reliable evidence of the geological period in which they were erupted. The lavas of the early Palæozoic ages may, indeed, on careful examination, be distinguished from those of Tertiary date, but, as we have seen, the difference is rather due to the effects of age and gradual alteration than to any inherent fundamental distinction between them. In all essential particulars of composition and internal structure, the lavas of the Cambrian or Silurian period resemble those of Tertiary and modern volcanoes. The igneous magmas which supply volcanic vents thus appear to have been very much what they are now from early geological epochs. At least no important difference, according to relative age, has yet been satisfactorily established among them. But although the rocks themselves afford no precise or trustworthy clue to their date, yet where they have been intercalated contemporaneously among fossiliferous stratified formations, of which the geological horizon can be determined from included organic remains, it is easy to assign them to their exact place in geological chronology. A determination of this kind is only an application of the general principle on which the sequence of the geological record is defined. A few illustrations will suffice to make this point quite obvious. Among the volcanic tuffs in the upper part of Snowdon various fossils occur, which are identical with those found in the well-known Bala Limestone. As the accepted reading of such evidence, we conclude that these tuffs must therefore be of the same geological age as that limestone. Now the position of this seam of rock has been well established as a definite horizon in the series of Lower Silurian formations. And we consequently without hesitation place the eruptions of the Snowdon volcano on that same platform, and speak of them as belonging to the Bala division of the Lower Silurian period. Again, in West Lothian the tuffs and lavas ejected from many scattered puys were interstratified among shales and limestones in which the characteristic fossils of the Carboniferous Limestone are abundant. There cannot, therefore, be any doubt that these eruptions were much younger than those of Snowdon, and that they took place at the time when the Carboniferous Limestone was being deposited. We thus speak of them as belonging to volcanoes which were active in that early part of the Carboniferous period to which the thick Mountain Limestone of Ireland and Derbyshire belongs.