yO PEOE. J. W. JUDD OK TEETIAEY 



mation to the o'phitic structure. The oliyine has become ahnost per- 

 fectly black and opaque by the development of magnetite particles in 

 it, and the residual material has been serpentinized. 



Fig. 6. More altered gabbro, Loch Ooruiskh, Isle of Skye. The felspar is 

 strikingly schillerized. along from one to five planes of the crystal. 

 The augite is schillerized in one set of planes parallel to the ortho- 

 pinacoid (diallage), or in two sets of intersecting planes, parallel to the 

 orthopinacoidand the clinopinacoid respectively (pseudo-hypersthene). 

 The olivine, which is small in quantity, is converted into serpentine 

 often penetrating into the cracks of surrounding minerals. The 

 relations of the augite and felspar-crystals suggest, as in the last ex- 

 ample, an approach to the ophitic structure. 



[Figs. 5 and 6 illustrate the more advanced conditions of alteration in these 

 rocks, by which they are made to resemble in their character the 

 gabbros (olivine-gabbros) of the older geological periods.] 



Fig. 7. Gabbro passing into dolerite, from Ardnamurchan. In this rock the 

 felspar, which is unweathered, but contains many inclusions, no longer 

 forms the large and broad crystals characteristic of the true gabbros, 

 but begins to assume long prismatic forms. The pale brown augite, 

 which is schillerized, and passes into diallage along the outer margins 

 of its crystals, shows a decided approach to the ophitic arrangement 

 in its relations to the felspars. The olivine forms irregular grains, 

 which exhibit various stages of the development of secondary mag- 

 netite in them. 



Fig. 8. Gabbro passing into dolerite, from near the summit of Halival, Isle of 

 Rum. In this very beautiful aud almost wholly unaltered rock the 

 breaking up of the broad crystals of felspar into " lath-shaped " forms 

 is carried still further. The rich green augite (diopside) forms rounded 

 grains, but these sometimes enclose lath-shaped felspar crystals. The 

 olivine, which also occurs in rounded grains, is very fresh, but con- 

 tains the stellar groups due to schillerization ; by slight alteration 

 along the cracks it has assumed a yellow-brown tint, as seen in the 

 fractured surface of the rock (see pp. 67, 68). 



From examples like those represented in figs. 7 and 8, we find the 

 most insensible transition to the true dolerite figured in Plate V. 



Plate V. 



DOLERITES PASSING INTO BasALTS. 



This Plate represents the structures of the dolerites and basalts and of the 

 rocks which constitute the transition between these two varieties. 



Figs. 1 to 4 are shown as magnified V) and Figs. 5 to 8 are shown as mag- 

 nified V. 



In this and the following Plate the rocks which have been formed by the 

 consolidation of materials in a state of internal equilibrium, are represented in 

 the column on the left hand, and those in which movement has taken place 

 during consolidation, on the right. 



Fig. 1. Ophitic structure in dolerite from the Shiant Isles. The section shows 

 dark purplish-brown augite, enclosing more or less lath-shaped crys- 

 tals of felspar, both minerals being beautifully fresh and unaltered. 

 A few scattered grains of olivine, slightly serpentinized along the 

 cracks, occur in the rock, with patches of magnetite ; and both of these 

 minerals sometimes enclose the felspars in the same way as does the 

 augite. Where weathering action has afiected the augite, it has 

 acquired the peculiar colour and pleochroism, but none of the other 

 characters of hornblende (see pp. 68, 85). 



Fig. 2. Granulitic structure in dolerite from Ardnamurchan. In this section 

 we find an interwoven network of lath-shaped felspar crystals, through 

 which irregular grains of pyroxene, olivine, and magnetite are scat- 

 tered, these grains being distinguished by their rounded contours and 



