328 
THE TERTIARY VOLCANOES 
BOOK VIII 
when my friend Professor Zirkel published the results of his tour through 
the A\ est of Scotland, and showed that the rocks in question were mostly 
time gabbros . 1 Since his observations were published some of these rocks 
have formed the subject of important papers by Professor Judd . 2 
The general petrographical characters of the gabbro areas of Western 
Scotland may be summarized as follows : — A very considerable variety of 
petrological structure and chemical composition is observable among the 
rocks. At the one end of the series are compounds of plagioclase and 
augite, which, though wanting in olivine, have the general structure and 
habit of dolerites. At the other end are mixtures wherein felspar is scarce 
or absent, and where olivine becomes the chief constituent. Between these 
two extremes are many intermediate grades, of which the most important 
are those containing the variety of augite known as diallage and also olivine. 
These are the olivine-gabbros, which form so marked a feature in the central 
parts of the great basic bosses. That some of these varieties of rock pass 
into each other cannot be doubted. Their distinctive composition and 
structure appear to have been largely determined by their position in the 
eruptive mass. The outer and thinner sheets are in great measure dolerites, 
with little or, no olivine. Coarse gabbros are abundant in the inner portions. 
Bocks rich in olivine, however, occur at the outer and especially the lower 
part of the gabbro masses of Bum and in some parts of Skye. The following 
leading varieties, may be enumerated : — 
Dolerite. — This rock varies from an exceedingly close grain (when it 
approaches and graduates into basalt) up to a coarse granular crystalline 
texture, in which the component minerals are distinctly visible to the naked 
eye. An average sample is found to consist of plagioclase, usually lath- 
shaped, and crystals or grains of augite with or without olivine. Under the 
microscope, the different varieties are distinguished by the presence of more 
or less distinct ophitic structure, the felspar being enveloped in the augite. 
kor the most part they are holocrystalline, but occasionally show traces of 
a glassy base. Ilmenite is not infrequent, with its characteristic turbid 
decomposition product (leucoxene). In other cases, the iron-ore is probably 
magnetite. Between the dolerites and gabbros no line of demarcation can 
be drawn in the field, nor can a much more satisfactory limitation be made 
even with the aid of the microscope. As a rule, the thickest and largest 
intrusive masses or bosses are gabbro, those of less size are dolerite, while 
the smallest (and sometimes the edges ol the others) assume externally the 
aspect of basalts. 
Gabbro. Under this term I arrange, as proposed by Professor Judd, 
all the coarse-grained granitoid basic rocks of the region without reference 
to the variety of augite present in them. Under the microscope, they are 
found to be holocrystalline, but with a granitic or granulitic rather than an 
ophitic structure, though traces of the latter are by no means rare. To the 
naked eye their component minerals are usually recognizable. Professor 
1 Zeitschrift. Deutsch. Geol. Gesellseh. xxiii. (1871), p. 1. 
2 Quart. Jo r. Geol. Soc. xli. (1885), p. 354 ; xlii. (1886), p. 49. 
