z hd , a + nd — 
ParrIl § vi] CRYSTALLINE-ROCKS—MASSIVE. 145 
as interstratified lava-beds, and in eruptive sheets, dykes, veins, and 
irregular bosses. In Scotland it forms masses, several thousand feet 
_ thick, erupted in the time of the Lower Old Red Sandstone, and 
_ others of wide extent, and several hundred feet in depth, belonging 
to the Lower Carboniferous period. In Germany it appears also 
at numerous points, where it is referred to later Palzozoic times. 
Porphyrite forms a connecting link between the hornblendic rocks 
and the augitic series next to be described. 
Diabase.—This name has been given to certain dark green or 
black eruptive rocks found in the older geological formations, and 
consisting essentially of triclinic felspar, augite, magnetite or titani- 
ferous iron, apatite, sometimes olivine, usually with more or less 
of diffused greenish substances (viridite) which have resulted from the 
alteration of the augite or olivine. The texture is sometimes quite 
crystalline; in other eases it shows a felsitic ground-mass. The 
average composition of typical diabase may be taken to be, silica, 
48—50; alumina, 16-0; protoxide of iron, 12—15; lime, 5—11; 
magnesia, 4—6; potash, 0‘S—1°5; soda, 3-4-5; water, 1-5—2, 
But there is generally carbonic acid present, united with some of the 
lime as a decomposition product. 
Diabase is sometimes exceedingly fine-grained and compact 
(diabase-aphanite) assuming aiso a fissile character (Diabas-schiefer), 
or taking a porphyritic structure, and showing dispersed crys- 
tals of the component minerals (diabase-porphyry, labrador-porphyry, 
augite-porphyry) ; or its ingredients, as In some varieties of diorite, 
assume a concretionary arrangement (variolite). When the green 
_ compact ground-mass contains small kernels of carbonate of lime, 
- sometimes in great numbers, it is called caleareous aphanite or eale- 
aphanite. Sometimes the rock is abundantly amygdaloidal. Though 
as a rule, free silica does net occur in it, some varieties have been found 
to contain this mineral, and are distinguished as guartz-diabase. 
Diabase occurs both in contemporaneous beds and in intrusive 
dykes and sheets. lt was formerly supposed to be confined to the 
older geological formations, while its place in Tertiary and recent 
times was taken by basalt. But some of the Miocene voleanie rocks 
of the west of Scotland are as good diabase as any among the Paleozoic 
formations; while, on the other hand, many of the dark heavy eruptive 
rocks belonging to the Carboniferous system in the basin of the Firth 
oi Forth are unquestionable basalts. The main difference between 
diabase and basalt appears to be that the rocks included under the 
former name have undergone more internal alteration, in particular 
acquiring the diffused “ viridite,” so characteristic of them.? 
Melaphyre.—This term has been so variously defined that the 
1 See an analysis of a porphyrite from the Vicentin, Von Lasaulx, Z Deutsch. 
Geol. Ges. xxv. p. 323. On microscopic structure of porphyrite of Ilfeld, see A. Streng, 
Neues Jahrb. 1875, p. 785. © 
? The student wili find in the Zeitschrift Deutsch. Geol. Ges., 1874, p. 1, an important 
memoir by Dathe on the composition and structure of diabase. See also Zirkel’s 
_ Mieroscop. Petrog. p. 97. — 
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