378 PKOF. J. AV. J ODD ON THE PROPYLITES 



2. At some points, esj)ecially near the east side of the mountain, 

 lava-currents composed of a compact rock are found. These, when 

 studied microscojncallj^, are seen to consist of a microlitic felt, in 

 which the largo porphyritic crystals are wholly wanting. The rock 

 is vesicular, and the contents of the vesicles appear to be an 

 altered glass. The specific gravity of this rock was found to be 

 2-89 (see Plate XV. fig. 1). 



3. Glassy {Viti-op7iy7^ic) Atidesite.— At certain points, especially 

 on the eastern side of the mountain, the rock is found to become 

 perfectly vitreous and to pass into a " pitchstone-porphyry." In 

 this the proportion of the glassy base to the porphyritic crystals 

 is sometimes very great, but the latter present all the characters of 

 the minerals found in the stony types of the rock (see Plate XV. 



It is interesting to note that in these glassy forms of the rock tlie 

 plagioclase crystals only show slight lamellar twinning in many 

 cases, and some of the types of the rock appear to approximate very 

 closely to the pitchstone-porphyry of the Sgiirr of Eigg. 



The glass sometimes exhibits the perlitic structure, and it varies 

 in density from 2-52 to 2*62. 



-4. When the glassy or microlitic felted base becomes small in 

 amount, it forms isolated masses which are caught up betw^een the 

 crystals, and the rock exhibits the " intersertal structure " of 

 E,osenbusch in a very striking manner, the rock becoming a 

 typical " tholeite " of that author. In many cases the glass of 

 these " tholeites " is crowded with skeleton-crystals of magnetite, 

 as in the case of the rocks figured and described by Mr. Teall * 

 (see Plate XV. fig. 4). 



5. Higlily Crystalline Andesites. — In places, especially in the 

 great central mass of the mountain, in some of the dykes, and in 

 the deepest part of the thick lava-streams, the rock loses almost all 

 trace of glass, and passes into a holocrystalline mass. These holo- 

 crystalline varieties sometimes exhibit the 0]iliitic structure ; while, 

 as in the case of the ophitic basalt of the same district, the 

 breaking up of the augite and felspar crystals into rounded gra- 

 nules, leads to a more or less perfectly developed gramditlc struc- 

 ture (see PI. XV. figs. 5 & 6). 



(). ]S"ew varieties make their appearance in consequence of 

 differences in the proportion of the several porphyritic constituents 

 to one another. These varieties are especially seen in the dykes, 

 some of which contain the plagioclase felspar almost to the exclu- 

 sion of the pyroxenes and magnetites ; while in other cases the 

 augite and magnetite are present in preponderating quantities, 

 and a rock of abnormal density and basicity is the result. (See 

 PI. XV., compare figs. 7 & S.) 



The wonderful variation in chemical composition which may 

 result from admixture in varying proportions of the same mineral 

 constituents is illustrated in the following table of analyses : — 



* Quart. Journ. Geol. Soc. vol. xl.;^(1884) pp. 209-246, pis. xii. & xiii. 



