650 SIR A. GEIKIE AND MB. J. J. H. TEALL ON THE [Nov. 1 894, 



crystals an inch or more in length. They occur as sheets, veins, 

 and more irregular masses, sometimes traversing and enclosing len- 

 ticles of fine-grained granulitic rock or cutting out parts of the 

 banding of the other gabbros. Their posteriority in such cases to 

 the two types of rock already described is thus made quite evident. 

 But where a bed of coarse massive gabbro lies between banded 

 sheets, with no visible evidence of transgression and no traceable 

 connexion with any mass which is transgressive, we are perhaps 

 hardly justified in speaking very positively as to its place in the 

 series of protrusions. All that can be definitely affirmed is that, 

 where the relative sequence of the rocks admits of determination, 

 the coarse massive forms are seen to have been protruded after the 

 first and second groups which have been here described. 



4. The Pale Veins. — These rocks, from their abundance and 

 their conspicuous whiteness, are prominent features all over Druim 

 an Eidhne, and indeed throughout the region of the Cuillin Hills. 

 They form irregular branching veins from several yards to less than 

 an inch in width, swelling out into thick aggregations and thinning 

 away into mere threads. Their whiteness is so much greater than 

 that of the pale bands in the gabbros, as to show that they contain 

 a large relative proportion of felspar with a smaller amount of 

 augite, olivine, and magnetite. That they belong to the gabbros as 

 part of one complete series of protrusions is recognized in the field, 

 even before the microscope demonstrates their relation to these rocks. 

 Yet they cannot be regarded as mere ' segregation-veins ' from the 

 rocks among which they rise. In the first place, the same vein may 

 be observed crossing successively examples of the dark fine-grained 

 sheets, the banded sheets, and the coarse massive gabbro. In the 

 next place, no sensible variation in composition and strncture can 

 be detected in these veins, as they strike from a rock rich in felspar 

 to one abounding in magnetite and the ferro-magnesian minerals. 

 They are undoubtedly intrusive where now visible, and, as they cross 

 all the other varieties of gabbro, they must be regarded as marking 

 the latest phase in the gabbro-protrusions of this locality. 



II. Microscopical and Chemical Characters or the Rocks. 



The Granulitic Gabbros. — In the fresh condition these are dark- 

 coloured, fine-grained, crystalline rocks composed of brown pyroxene, 

 water-clear felspar, green pseudomorphs, and magnetite. 



The pyroxene occurs in grains of nearly equal dimensions in the 

 different directions (*1 to "2 millim.). It is sometimes in the 

 condition of ordinary augite, but more frequently contains the inclu- 

 sions characteristic of diallage and pseudo-hypersthene. 



The grains of felspar resemble those of pyroxene in form and size. 

 Twinning is not uncommon, but the bands are few in number. 

 The extinction-angles point to a variety allied to labradorite. Minute 

 inclusions containing bubbles may be observed with a high power. 



The green pseudomorphs agree in form and size with the grains 



