When the conditions of modern volcanic eruptions are considered, it will be seen that where ejected ashes and stones fall into water, they will there mingle with any ordinary sediment that may be in course of deposition at the time. There will thus be a blending of volcanic and non-volcanic detritus, and the transition between the two may be so insensible that no hard line of demarcation can be drawn. Such intermingling has continually taken place during past ages. One of the first lessons learnt by the geologist, who begins the study of ancient volcanic records, is the necessity of recognizing this gradation of material, and likewise the frequently recurring alternations of true tuff with shale, sandstone, limestone or other entirely non-volcanic detritus (Fig. 14). He soon perceives that such facts as these furnish him with some of the most striking proofs of the reality and progress of former eruptions. The intermingling of much ordinary detritus with the volcanic material may be regarded as indicative either of comparatively feeble activity, or at least of considerable distance from the focus of discharge. It is sometimes possible to trace such intermixtures through gradually augmenting proportions of volcanic dust and stones, until the deposit becomes wholly volcanic in composition, and so coarse in texture as to indicate the proximity of the eruptive vent. On the other hand, the gradual decrease of the volcanic ejections can be followed in the upward sequence of a series of stratified deposits, until the whole material becomes entirely non-volcanic. The occurrence of thin partings of tuff between ordinary sedimentary strata points to occasional intermittent eruptions of ashes or stones, the vigour and duration of each eruptive interval being roughly indicated by the thickness and coarseness of the volcanic detritus. The pauses in the volcanic activity allowed the deposit of ordinary sediment to proceed unchecked. The nature of such non-volcanic intercalations gives a clue to the physical conditions of sedimentation at the time, while their thickness affords some indication of the relative duration of the periods of volcanic repose. A little reflection will convince the observer that in such a section as that represented in Fig. 14 the volcanic intercalations must be regarded as a mere local accident. Evidently the normal conditions of sedimentation at the time these strata were accumulated are indicated by the limestone bands (_l_, _l_). Had there been no volcanic eruptions, a continuous mass of limestone would have been deposited, but this continuity was from time to time interrupted by the explosions that gave rise to the intercalated bands of tuff (_t_, _t_). The application of these rules of geological evidence will be best understood from actual examples of their use. Many illustrations of them will be subsequently given, more especially from the volcanic records of the Carboniferous period.