Part Il. § vi] CRYSTALLINE ROCKS—MASSIVE. 149 
that the individual crystals are scattered widely through it or drawn 
out into beautiful streaks and eddies of fluxion structure, in others it 
almost or wholly disappears, and the rock then appears as a nearly 
or quite crystalline mass. 
Vitreous Basalt. (Tachylite, Hyalometan.) In some cases 
basalt passes into a condition which, even to the naked eye, is 
recognizable as that of a true glass. This more especially takes: 
place along the edges of dykes and intrusive sheets. Where an 
external skin of the original molten rock has rapidly cooled and 
consolidated in- contact with the rocks through which the eruption 
took place, a transition can be traced within the space of less than 
a quarter of an inch from a crystalline dolerite, anamesite or basalt, 
into a black glass, which, under the microscope, assumes a pale 
brown or yellowish colour, and is isotropic, but generally contains 
abundant microliths; sometimes with a globular or spherulitic 
concretionary strueture. In such cases it seems indisputable that 
this glass represents what was the general condition of the whole. 
molten mass at the time of eruption, and that the present crystal- 
line structure of the rock was developed during cooling and con- 
solidation. It is worthy of remark that in the analyses of vitreous 
basalts the percentage of silica rises usually above that of ordinary - 
crystalline basalt. The average composition of the basalt rocks is 
shown in the subjoined Table: 










Oxides of | Loss by 
sa 3 : - | Eronsand : 2a ignition | Specific: | 
Silica. | ing, | Lime. |Magnesia.) j70, gan- Potash. | Soda. (water, | gravity. 
ese. &c.). 
Dolerite . . |45—55|12—16| 7—13 3—9 9—18 0—1 2—5 O°5—3 | 2°75—2°96 
Anamesite. . | 46—53}12--15| 8°5—13 |1°5—9°5| 10—15 0°5—1 2—3 1—3 Z:°7—2°8 
asalt . . . |45—55/10—18| 7—14 | 3—10 9-16 sie Ors——Sal) 25 1—5 | 2°*85—3°10 
Vitreous Basalt | 48—58 | 12—17 | 6°5—9°5| 0°5—6 8—20 | 0°5—9°5| 2°5—5 | 0°5—3°5|) 2°6—2°7 
The basalt-rocks are thoroughly volcanic rocks, appearing in lava- 
streams, sheets, plateaux, dykes, and veins. The finer grained 
varieties are often beautifully columnar; hence the term “ basaltic ” 
has been popularly used to denote the columnar structure. Porphy- 
ritic and amygdaloidal varieties are of frequent occurrence. 
As already stated, it has been assumed by some writers that 
basalt did not. begin to be erupted until the Tertiary period. But 
true basalt occurs abundantly in Scotland, as a product of Jower 
Carboniferous volcanoes. There seems, however, to be no doubt 
that, as: Richthofen first pointed out, in the order of appearance at any 
given volcanic focus, basalt comes up after the rhyolitic and trachytic 
eruptions have ceased. (See Book III. Part I. Section i. § 5.) 
Zirkel has divided basalt into felspar-basalt, which is the rock 
now described; nepheline-basalt and leucite-basalt. The two latter 
rocks, in which the part of the felspar is played by nepheline and 
_leucite respectively, are enumerated on the next page. 
Gabbro (Diallage Rock) is a thoroughly crystalline granitoid 
