GABBKOS ETC. IX SCOTLAND AKD lEELAXD. 69 



The structures which we have now described belong to the " holo- 

 crystalline" rocks of Professor Eosenbusch, the "type granitoide " 

 of M. Michel-Levy. The hemicrystalline rocks, rocks in which 

 only a part of the magma has assumed the crystalline form before 

 consolidation (" porphyrische Gesteine" in part of Rosenbusch, and 

 "type trachytoi'de" of Michel-Levy), have now to be considered. 



If to the grauulitic or orphitic types of these rocks there be added 

 a greater or smaller amount of vitreous ground-mass, we have the 

 structure which characterizes the basalts. It may be convenient, 

 with Mohl and Boricky, to separate as a subgroup the varieties in 

 which the qug,ntity of vitreous or semivitreous base is excessive, 

 under the name of " magma-basalts," these magma-basalts gra- 

 duating insensibly on the one hand into ordinary basalts, and on 

 the other into basalt-glass. 



When we study with care the micro-structure of this more or less 

 perfectly vitreous base — which forms the mass of basalt-glass, a 

 large proportion of the magma-basalts and a smaller proportion of 

 the ordinary basalts — we are struck with the fact that two 

 strikingly distinct types of structure are recognizable among its 

 numerous varieties. 



In the first of these types we find the glassy ground-mass filled 

 with opaque skeleton-crystals of magnetite and numerous long 

 transparent rods often bifurcating at their ends ; these appear to 

 be skeleton-crystals of either augite or felspar, or of both those 

 minerals. When the development of this structure has gone a little 

 further it is often possible to identify the skeleton-crystals as those 

 either of augite or of felspar. These opaque and transparent 

 skeleton- crystals are seen entangled with one another in every 

 position, and forming a crystalline network within the glass (see 

 Plate yi. figs. 1, 3, 5, 7). 



In the second type of vitreous base we find the opaque material 

 in the form of cubical or more or less rounded grains, either isolated 

 or irregularly aggregated. The felspar usually forms small and short 

 microlites, the ends of which often exhibit step-like terminations, 

 and the augite, when individualized, forms more or less rounded 

 granules (see Plate YI. figs. 2, 4, 6, 8). 



This distinction between the types of the glassy ground-mass in 

 basalts can be made out alike in cases where the well-developed 

 crystals are numerous, and in those in which they are comparatively 

 few in number (magma-basalts). It is equally well seen, though 

 developed on a more minute scale, in the basalt-glasses, some of 

 which, as shown by Mr. Cole and myself, are filled with magnetite 

 " dust," while others exhibit skeleton-crystals of magnetite and 

 sometimes of transparent minerals *. 



Porphyritic varieties are found in the case of all the different 



types of these rocks, from the most perfectly holocrystallinc to those 



which are absolutely vitreous. While the rocks of this area are 



rendered " porphyritic " by the presence of scattered crystals of 



* Quart. Journ. Geol. Soc. vol. xli. (1885) p. 444. plate xiv. 



