DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



find that those which run in one direction are invariably cut by those which run in 

 another, the inference can hardly be resisted that they do not belong to the same eruption, 

 but mark successive epochs of volcanic energy. An excellent example of this kind of 

 evidence is furnished by Mr Clough from eastern Argyleshire. The east and west dykes in 

 that district are undoubtedly older than those which run in a N.N.W. direction (fig. 17). 

 The latter are by far the most abundant, and are on the whole much narrower, less per- 

 sistent, and finer in grain. On the opposite coast of the Clyde, a similar double set of 

 dykes may be traced through Renfrewshire, those in an east and west direction being com- 

 paratively few, while the younger N.N.W. series is well developed. The great sheets or 

 " sills" connected with one of the Stirlingshire dykes, already described, appear to me to 

 furnish similar evidence in the younger- dykes which run through them. And this 

 evidence is peculiarly valuable, for it shows a succession even among adjacent dykes 

 which all run in the same general direction. 



But in all these cases it is obvious that we have little indication of the length of time 

 that intervened between the successive uprise of the dykes. In Skye, however, we meet 

 with more definite evidence that the interval must have been in some cases a protracted 

 one. In a paper published as far back as the year 1857,* I showed that the basic dykes 

 of Strath in Skye were of two ages ; that one set had been erupted before the appearance 

 of the so-called " syenite " of that district, and was cut off by the latter rock ; and that 

 the other had arisen after the ** syenite " which it intersected. Recent re-examination 

 has enabled me to confirm and extend this observation. The granitoid eruptions of the 

 Inner Hebrides are marked by so varied a series of rocks, and so complex a geological 

 structure, that they may, with some confidence, be regarded as having occupied a 

 considerable interval of geological time. Yet we find that this episode in the volcanic 

 history was both preceded and followed by the extravasation of basic dykes. I have been 

 unable to make out any appreciable petrographical difference between the two sets of 

 dykes. But for the evidence of the granophyre, they would unquestionably be all classed 

 together as one series. 



Let me add one further piece of evidence to prove that some of the dykes go back to 

 a remote part of the volcanic history of Tertiary time in Britain. The Scuir of Eigg, to 

 which fuller reference will be made in a later part of this memoir, is formed of a mass 

 of pitchstone, which has filled up an ancient valley eroded out of the terraced basalts of 

 the plateaux. At both ends of the ridge, these basalts are seen to be traversed by dykes 

 that are abruptly cut off by the shingle of the old river-bed which the pitchstone has 

 occupied (fig. 63). It is thus evident that, though these dykes are younger than the 

 plateau-basalts, they are much older than the excavation of the valley out of these basalts, 

 and still older than the eruption of pitchstone. The latter rock probably belongs to the 

 close of the period of acid eruptions just referred to, and we have seen that abundant 

 dykes were extruded after most of the acid rocks had appeared. 



It is certain, therefore, that the dykes which in Britain form part of the great Tertiary 



* Quart. Jour. Geol. Soc, xiv. p. 1. 

 VOL. XXXV. PART 2. K 



