DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 37 



While, as a general rule, the external portions of a dyke are closer-grained than the 

 centre, rare cases occur where the middle is the most finely crystalline part. I am 

 disposed to regard these cases and the glassy centres as forming in reality no true 

 exceptions to the rule, that the outer portions of a dyke consolidated first, and are there- 

 fore finest in texture. For the most part, each dyke appears to be due to a single uprise 

 of molten matter, though considerable movements may have taken place within its mass 

 before the whole stiffened into stone. But where, after more or less complete consolidation 

 had taken place, the fissure opened again, or from any other cause the dyke was split along 

 its centre, any lava which rose up the rent would tend to take a finer grain than the 

 material of the rest of the dyke, and might even solidify as glass. 



Large scattered crystals of felspar, of an earlier consolidation than that of the minuter 

 forms of the same mineral in the general ground-mass of the rock, give a porphyritic 

 structure and andesitic character to many dykes. Occasionally such crystals attain a 

 considerable size. Mr Clough has observed them in some of the Argyleshire dykes 

 reaching a length of between three and four inches, with a thickness of two inches. Some- 

 times they are distributed with tolerable uniformity through the substance of the dyke. 

 But not infrequently they may be observed in more or less definite bands parallel with 

 the boundary walls. Unlike the younger lath-shaped and much smaller felspars of the 

 ground-mass, they show no diminution either in size or abundance towards the edge of the 

 dyke. On. the contrary, they are often conspicuous in the close-grained marginal strip, 

 and they may be found even in the glassy selvage, and touching the very wall of the 

 fissure. Indeed, they are sometimes more abundant in the outer than in the inner 

 portions of a dyke. 



Mr Clough has given me the details of an interesting case of this kind observed by 

 him in Glen Tarsan, Eastern Argyleshire : — " For an inch or so from the edge of this 

 dyke," he remarks, " porphyritic felspars giving squarish sections, and ranging up to one- 

 third of an inch in length, are so abundant as nearly to equal in bulk the surrounding 

 ground-mass. For the next inch and a half, they are decidedly fewer, occupying perhaps 

 hardly an eighth of the area exposed. Then for a breadth of three inches they come in 

 again nearly as abundantly as at the sides ; after which they diminish through a band 

 27 inches broad, where they may form from -^ to ^ of the rock." He found another 

 case where, in a dyke several yards wide, porphyritic felspars, sometimes an inch long, 

 are common along the eastern margin of the dyke in a band about two inches broad, 

 but are nearly absent from the rest of the rock. Elsewhere the crystals are grouped 

 rather in patches than in bands. 



Not only are these porphyritic felspars apt to occur in bands parallel with the outer 

 margins of the dykes, but they tend to range themselves with their longer axis in the 

 same direction, thus even on a large scale, visible at some distance, showing the flow- 

 structure, which is so often erroneously regarded as essentially a microscopic arrange- 

 ment. 



Another macroscopic character of the material composing the dykes is the frequent 



VOL. XXXV. PART 2. F 



