DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 127 



green slopes of the basalt-plateau, rising platform above platform to a height of -nearly 

 1500 feet above the sea. But turning to the east, he beholds the dark, gloomy, cauldron- 

 like Corry na Creiche, from which rise some of the ruggedest and loftiest crests of the 

 Cuillins. On the hills that project from either side of this recess and half inclose it, 

 the bedded basalts mount from the bottom of the valley, with their lines of parallel terrace 

 dipping gently inward below the black rugged gabbro that crowns them and sweeps 

 round to form the back or head of the corry. Down the whole length of Glen Brittle 

 the same structure conspicuously governs the topographical features. On the right hand, 

 the ordinary terraced basalts form the slopes ; and they rise for some 500 or 600 feet up 

 the eastern side, until they pass under the darker, more rugged, and less distinctly bedded 

 rocks of the mountains (fig. 36). The dip of the whole series is here at a gentle angle 

 toward S.E. , or into the main mass of the Cuillin group. 



When, however, we proceed to examine the junction between the two rocks we find 

 it to be less simple than it appears. It is not an instance of mere superposition. The 

 gabbro unquestionably overlies the basalts, and is therefore of younger date. But it 

 overlies them, not as they rest on each other, in regular conformable sequence of eruption, 

 but intrusively, as a sill does upon the rocks on which it appears to follow in the unbroken 



Fig. 36. — Section across Glen Brittle, to show the general relations of the Bedded Basalts (a) and the Gabbros (6). 



order of accumulation. This important structure may be ascertained in almost any of 

 the many sections cut by the torrents which have so deeply trenched with gullies the 

 flanks of the hills. Starting from the ordinary bedded basalts, we observe in mounting 

 the slopes and approaching the gabbro that the rocks insensibly assume that indurated 

 sbattery character, which has been referred to as characteristic of them round the 

 margins of vents. Beds of dolerite make their appearance among them, which are so 

 distinctly crystalline, and so resemble in character the rocks of the sills, that there can be 

 little hesitation in regarding them as intrusive. These sills increase in size and number 

 as we ascend, though hardened amygdaloidal basalts may still be observed. True gabbros 

 then supervene in massive beds, and at last we find ourselves entirely within the gabbro 

 area, where, however, thin bands of highly altered basalt still for some distance appear. 

 One further fact will generally be noticed, viz., that before reaching the main mass of 

 gabbro, veins and sills of basalt, as well as of various felsitic and porphyritic members of 

 the acid group, come in abundantly crossing and re- crossing each other in the most intri- 

 cate network. The base of the thick gabbro-sheets is thus another horizon on which, 

 like that below the plateau-basalts, intrusive masses have been especially developed. 



