DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 119 



Ben Hiant the individual crystals reach an inch or more in length. These more largely 

 crystalline portions, however, do not form distinct bands so much as patches in the midst 

 of the general mass ; at least I did not notice any examples of such veins of segregation 

 as are so prominent in Antrim. 



Owing to the denudation, which has laid this mass bare, no trace now remains of the 

 rocks which covered it. That it is really an intrusive mass and not a superficial outflow, 

 as Professor Judd supposed, may be inferred from the way it slants across the 

 beds of the plateau, and also from its petrographical characters, in which it agrees 

 with other undoubtedly intrusive rocks of the same series. None of the rocks which 

 unquestionably flowed out on the surface present this coarsely crystalline structure. On 

 the south-east of Ben Hiant, I observed at two places where the bedded basalts could be 

 traced close up to the intrusive mass that the former presented the same dull indurated 

 character which I have referred to as occurring near intrusive bosses on the plateaux. 

 As shown on the small map in Plate I., two marked dykes diverge from the main mass 

 of dolerite and run for some distance north-eastward. One of these, fully a mile long, 

 descends into the valley and rises up into the basalt-plateau on the further side. 

 Possibly these may be two of the feeders from which the thick mass of Ben Hiant was 

 supplied. 



But infinitely more frequent in the west of Scotland is the second type of intrusive 

 sheet, where, instead of lying in lenticular, coarsely-crystalline sills, the eruptive rock 

 occurs in thin sheets, interposed with sometimes most deceptive regularity between the 

 bedding planes of the rock which it traverses. It is this type which has become so familiar, 

 from the descriptions and sections of Maccdlloch, as characteristically given in his 

 account of Skye and in his drawings of the east coast of Trotternish in plate xvii. of 

 his illustrations already cited. The height to which the base of the basalt escarpment 

 rises on that side of the island, and the fine range of cliffs which there underlie it, permit 

 the phenomena of intrusive sheets to be studied better than perhaps anywhere else in 

 Britain. The erupted material has been thrust between the stratification planes of the 

 Jurassic strata, between the strata and the overlying bedded basalts, and between the 

 individual sheets of basalt. As Macculloch well shows, many sheets of intrusive rock, 

 if seen only at one point, might readily be supposed to be regularly interstratified ; but 

 perhaps only a few yards distant they may be found to break across the bedding, and to 

 resume their course on a different level. 



The phenomena of the eastern coast of Trotternish are repeated on the east side of 

 Eaasay, in Eigg, and in Mull. As a single example of them, I may select the accompany- 

 ing section (fig. 35) from the east side of Eigg. Over the Jurassic sandstones (aa) a 

 sheet of basalt (1) four to six feet thick has been injected between the stratification of 

 the sandstones, and another (2) two to four feet thick has forced its way across the 

 middle of one of the bedded basalts (bb) in which it bifurcates, and above which comes the 

 thick series of lavas of the plateau (c, d). In one of the streamlets, which exposes a 

 section of the Jurassic strata below the volcanic escarpment, more than twenty intrusive 



