DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 29 



subject, I offer my results to the Royal Society of Edinburgh, which honoured and 

 encouraged me by printing in its Transactions my first essay on the volcanic rocks of 

 this country. 



In describing the geological history of a great series of rocks, chronological order is 

 usually the most convenient method of treatment. Where, however, the rocks are of 

 volcanic origin, and do not always precisely indicate their relative age, and where more- 

 over the same kinds of rock may appear on widely separated geological horizons, it is 

 not always possible or desirable to adhere to the strict order of sequence. With this 

 necessary latitude, I propose to follow the chronological succession from the older to the 

 newer portions of the series. I shall treat first of the system of basic dykes, by which so 

 large a part of Scotland and of the north of England and Ireland is traversed. Many of 

 the dykes are undoubtedly among the youngest members of the volcanic series, and in 

 no case can their age be determined except relatively to the antiquity of the rocks which 

 they traverse. They must, of course, be posterior to these rocks, and hence it would be 

 quite logical to reserve them for discussion at the very end of the whole volcanic 

 phenomena. My reason for taking them at the beginning will be apparent in the sequel. 

 After the dykes, I shall describe the great volcanic plateaux which, in spite of vast 

 denudation, still survive in extensive fragments in Antrim and the Inner Hebrides. The 

 eruptive bosses of basic rocks that have broken through the plateaux will next be discussed. 

 An account will then be given of the protrusions of acid rocks which mark the latest 

 phase of eruption in the region. The last section of the memoir will contain a summary 

 of the history of Tertiary volcanic action in Britain. 



I. THE BASIC DYKES. 



If a geologist were asked to select that feature in the volcanic geology of the British 

 Isles which more than any other marks this region off from the rest of the European 

 area, he would probably choose the remarkable system of wall-like masses of erupted 

 igneous rock to which the old Saxon word " dykes" has been affixed. From the moors of 

 eastern Yorkshire to the Perthshire Highlands, and from the basins of the Forth and Tay 

 to the west of Donegal and the far headlands of the Hebrides, the country is ribbed across 

 with these singular protrusions to such an extent that it may be regarded as a typical 

 region for the study of the phenomena of dykes. That all the dykes in this wide tract 

 of country are of Tertiary age, I am far from believing. Some of them are of the era of 

 the Old Red Sandstone, others are undoubtedly Carboniferous, while some, though later than 

 the Coal Measures, may be older than the Permian, or at least the Trias formations. As 

 illustrations of these older dykes, I may refer to the remarkable series which traverses the 

 Carboniferous rocks of Northumberland and Durham, and which includes the well-known 

 Whin Sill. That this series belongs to a totally different and greatly more ancient period 

 of extrusion was indicated many years ago by Sedgwick,* who referred to the previous 



* Trans. Cambridge Phil. Soc. (1822), vol. ii. p. 23; Wincb, Geol. Trans., iv. (1814) p. 25. 

 VOL. XXXV. PART 2. E 



