DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 123 



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. The coarse gabbros are found in the inner portions. Eocks rich in olivine, 

 however, occur at the outer and especially the lower part of the gabbro boss of Eum. 

 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. 

 For the most part they are noncrystalline, 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 of the others) assume 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 rather than an ophitic structure, though traces of the latter are by no means 

 rare. To the naked eye their component minerals are usually recognisable. Professor 

 Zirkel, from his examination of the Mull gabbros, believed them to consist of three 

 parts of plagioclase, two parts of olivine, and one part of diallage.* Olivine, however, is 

 not invariably present. t The pyroxene also does not always show the peculiar fibrous 

 structure of diallage. Professor Judd, indeed, maintains that the diallagic form is due to 

 a deep-seated process of alteration (schillerization), and that the same crystal may consist 

 partly of ordinary augite and partly of diallage.J Ilmenite (with leucoxene), magnetite, 

 apatite, biotite, and epidote are not infrequent constituents. 



Troctolite (Forellenstein). — This beautiful variety of plagioclase-olivine rock occurs as 

 a conspicuous feature on the east side of the gabbro-boss of the island of Eum. It forms 

 a bed on the side of the mountain Allival, in which the component minerals are drawn 



* Zeitschr. Deutsch. Geol. Gesellsch., xxiii. (1871), p. 59. 



t Professor Judd (Quart. Jour. Geol. Soc, xlii p. 62) believes that originally all the gabbros contained olivine, 

 and that where it is now absent, it has been altered into magnetite or serpentine. 



% Op. cit., xli. In a later paper he insists on the gradation of the coarse granitoid varieties (gabbros) into 

 holocrystalline compounds, where the felspar appears in lath-shapes with crystals or rounded grains of augite and 

 olivine (dolerites), and thence into true basalts, magma-basalts, and tachylytes (op. cit, xlii. p. 62). 



