DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 115 



margins of the eruptive masses at Portrush, is strikingly exhibited at Fair Head. For 

 about 18 or 20 inches upward from the bottom, where the bed rests on the black, 

 Carboniferous shales, the dolerite is dark and finely crystalline, weathering spheroidally 

 in the usual manner. But immediately above that bottom layer of closer grain the 

 normal coarsely crystalline texture rapidly supervenes. A similar closeness of grain 

 is observable at the surfaces of contact where the sheet splits up on its western 

 border. 



Nowhere, so far as I know, can the phenomena of segregation veins be so instructively 

 studied as along the abundant exposures of this great sheet. The veins are most conspicu- 

 ous where the rock occurs in thickest mass. They vary up to three or four feet in thick- 

 ness, and, as at Portrush and elsewhere, lie on the whole parallel to the upper and under 

 surfaces of the sheet. An erroneous impression may be conveyed by the term " veins " 

 applied to them. They are quite as much layers, parallel on the whole with the bedding 

 of the sheet, yet not adhering rigidly to one plane, but passing across here and there 

 from one horizon to another. That they are not due to any subsequent protrusion of 

 younger material through the main sheet is made manifest by the thorough interlocking 

 of their component crystals with those of the body of the rock in which they lie. They 

 consist of an exceedingly coarse aggregate of crystals, or rather of crystalline lumps of 

 the minerals that constitute the general mass of the rock, the felspar and augite showing 

 the ophitic intergrowth of the main rock, but on a far larger scale. Some of the pieces 

 of augite measure two inches or more in diameter. 



This great Fair Head sheet lies upon Carboniferous strata, but that it is to be classed 

 with the Tertiary volcanic series is, I think, demonstrated by its relations to the Chalk at 

 its eastern end. It has there broken through that rock, and converted it for a short 

 distance into a white, granular marble. But it is at the western side that the most 

 interesting sections occur to show the truly intrusive nature of the mass. The rock there 

 splits up into about a dozen sheets, which, keeping generally parallel with each other, have 

 forced their way between and partly across the bedding planes of the Carboniferous 

 shales (fig. 34). In this way the huge, unbroken mass, 250 feet thick, subdivides itself 

 and disappears in a few hundred yards, though it continues a little further inland, and 

 approaches the shore again half a mile to the south-west. Further evidence of the 

 intrusive nature of this rock may be observed along the base of the precipice, where at 

 least one sheet 70 feet thick diverges from the main mass and runs eastwards between 

 the Carboniferous shales (fig. 33). At the contact with the eruptive rock the shales are 

 everywhere much indurated. 



All through the Inner Hebrides the base of the basalt series generally presents 

 abundant examples of intrusive sheets. I have already alluded to this fact as an 

 explanation of the conclusion to which geologists were led, that in Skye and elsewhere 

 the basalts are interstratified with the Jurassic rocks, and are consequently of Jurassic age. 

 It was Macculloch who first described and figured in detail these proofs of intrusion. 

 His well-known sections in plate xvii. of the illustrations of his work on the Western 



