^2 PEOF. J. W. JIJDD ON- TERTIARY 



greater. The magnetite forms large irregular grains. This type 

 admirably illustrates the transition between the porphyro-granulitic 

 dolerites and the most common type of basalts forming the lava- 

 streams of the district, in which minute granules of augite and olivine 

 with ill-defined lath-shaped microlites of felspar are scattered through 

 a vitreous base. See Plate VI. fig. 2. 



Plate YI. 



Basalts passing into Magma-basalts. 



The first four figures on this Plate are shown as seen with a magnifying 

 power of 150 diameters (H-); the remaining four with a magnifying power of 

 300 diameters (^f ^). 



Fig. 1. Compact basalt from a dyke at Carsaig, Isle of Mull. The glassy base 

 is filled with beautiful rods and skeleton- crystals of magnetite ; irregular 

 and ill-defined microlites of felspar are seen more or less enclosed (in 

 the ophitic manner) in the also somewhat ill-defined augite-microhtes. 

 The scattered grains of oliviue are completely serpentinized. This 

 section illustrates the combination of an imperfect ophitic structure 

 with one in which skeleton-crystals are conspicuous, both types 

 characterizing the basalts which have consolidated with perfect 

 internal equilibrium. 



Kg. 2. Compact basalt of the Giant's Causeway, Co. Antrim, Ireland. 

 Irregular lath-shaped microlites of felspar with granular inicroHtes 

 of augite and olivine are set in a small quantity of a glassy base, fiilled 

 with granular particles of magnetite, and similar granules and 

 giobulites of magnetite are sometimes found enclosed in the augite- 

 and felspar-microlites. This rock illustrates the common type of the 

 more compact basalts of the area. 



Fig. 3. Compact basalt from near Tobermory, Isle of Mull. In this rock such 

 a large quantity of glassy base is present that it might perhaps almost 

 be considered as a magra a-basalt. Through the glassy base are scattered 

 needles of felspar and augite, the latter sometimes aggregated into 

 fibrous and plumose masses, which enclose the former with very minute 

 black rods and granules of magnetite. The structure exhibited is very 

 similar to that of some of the " light magma-basalts " of Boricky . 



Fig. 4. Compact basalt from Lismagogue, Co. Antrim, Ireland. This variety 

 differs from that of fig. 2, chiefly in the larger quantity of glassy base 

 thickly set with granules of magnetite. The same irregular microlites 

 of felspar, with the rounded granules of augite, are found in this rock, 

 as is so commonly the case with the basalts of lava-streams. The 

 olivine which is small in quantity, is completely serpentinized. The 

 rock approaches in its characters the " dark magma-basalts " of 

 Boi-icky. 



Fig. 5. Magma-basalt from a dyke at the Beal, near Portree, Isle of Skye. The 

 structure of this rock resembles that of fig. 3, but the crystallization 

 of the several mineral constituents has not gone so far. Rods of 

 magnetite and irregular lath-shaped microlites of felspar are scattered 

 through the mass. The enclosing glass is filled with minute crystal- 

 lites of augite which are only very occasionally seen to be united into the 

 fibrous and plumose masses found in fig. 3. The glass crowded with the 

 augite, crystallites encloses the felspar microlites in the same way that 

 the augite does in the ophitic dyke of rocks. It is clear that fig. 5 

 represents a type of rock in which a larger part of the material remains 

 in a vitreous condition than thai represented in fig. 3, and one 

 in which the crystallization of the minerals, especially of the augite, has 

 not gone so far. 



Fig. 6. Magma-basalt from near Tobermory, Isle of Mull. This rock resembles 

 the common type represented in figs. 2 and 4 except for the fact that 

 the bulk of the material remains in the glassy condition. The 



