292 PKOCEEDIKSS OF THE GEOLOGICAL SOCIETY. [-^-pr. 5, 



del Annunziata. In some cases the slaggy upper part of one bed 

 and the corresponding lower part of the bed above it seem to pass into 

 each other, although the general bedded structure of the whole re- 

 mains very marked at a little distance. Here and there, as at the 

 north end of Beinn Bhuidh, illustrations are afforded of the elonga- 

 tion of the cavities along the upper surface, showing the direction 

 in which the lava was moving before it finally cooled and con- 

 solidated. 



2nd. Some of the beds are coarsely amygdaloidal throughout. In 

 the kernels are found the usual minerals which result from the de- 

 composition of basalt-rocks — mesotype, stilbite, calc-spar, amethyst, 

 chalcedony, quartz-crystals, &c. And it is to be remarked, in Eigg 

 as elsewhere throughout the "Western Islands, that the abundance 

 of the amygdaloidal minerals is proportioned to the amount of alter- 

 ation which has been undergone by the general matrix of the rock 

 in which they lie. 



3rd. Although the interbedded sheets are sometimes seen to die 

 out along the line of cliff, they never penetrate or otherwise dis- 

 turb each other. This feature is one which has not been recognized 

 by previous writers on the igneous rocks of the Inner Hebrides. It 

 has been lost sight of among the proofs of intrusion furnished by so 

 many of the basaltic sheets ; and thus the "trap" or "overlying 

 rocks " of Skye and the other islands have come to be regarded as 

 typical examples of intrusive igneous masses, and described and 

 figured as such in innumerable text-books. Yet no fact is more 

 absolutely certain than that the vast mass of the basaltic rocks of 

 these regions consists of interbedded sheets, which flowed out, 

 one over another, at the surface, and have no intrusive characters. 

 They are traversed, however, by intrusive sheets and dykes, as will 

 be pointed out in the sequel. 



4th. The occurrence of intercalated tuffs, volcanic breccias, and 

 layers of burnt soil in Eigg, and of shales with remains of land- 

 plants and seams of coal in the other islands, completes the proof 

 that the basaltic beds forming the great plateaux, must be regarded 

 as of interbedded or contemporaneous origin — that is, sheets which 

 were poured out as lava above ground, and not injected among older 

 rocks below. 



/3. Porphyrife. 



Under this tenn I include a well-marked bed, forming a conspi- 

 cuous band along the range of cliffs which flank the plateau of 

 Beinn Bhuidh (see figs. 2 and 3). It lies near the base of the 

 volcanic series. Owing to the flatness of the beds and to denuda- 

 tion, it has been uncovered, so as to stretch over most of the bottom 

 of the hollow between Kildonan and the Bay of Laig. But I did 

 not find it in the southern half of the island. This rock is of a pale 

 grey colour. It consists of a finely crystalline felspathic base, through 

 which a few small plagioclase crystals and grains of titaniferous 

 iron can be seen with the lens. Examined with the microscope by 



