THE TERTIARY VOLCANOES 
BOOK VIII 
I84 
in great perfection. Many of the rocks are holo-crystalline, but usually 
show more or less interstitial matter ; in others the texture is finer, and the 
interstitial matter more developed ; in no case, as far as I have observed, 
are there any glassy varieties, which are restricted to the dykes and sills, 
though in some of the basalts the proportion of glassy or incompletely 
devitrified substance is considerable. The felspars are generally of the 
characteristic lath-shaped forms, and are usually quite clear and fresh. 
The augite resembles that of the dykes, occurring sometimes in large plates 
that enclose the felspars, at other times in a finely granular form. Olivine 
is frequently not to he detected, even by green alteration products. 
Magnetite is sometimes present in such quantity as to affect the compass of 
the field-geologist. Porpliyritie varieties occur with large felspar pheno- 
crysts ; but such varieties are, I think, less frequent among the plateau-rocks 
than among the dykes. They are well developed in the west part of the 
island of Canna, and have been described from the Faroe islands. Occasion- 
ally the plateau lavas are full of enclosed fragments of other rocks which 
have been carried up in the ascending magma. 
(&) Andesites and Trachytes . — Probably the majority of these rocks where 
they occur intercalated between the basalts of the plateaux are, as already 
remarked, intrusive sheets rather than true lavas. But they have also been 
poured out intermittently among the basalts and dolerites. The most ex- 
tensive development of lavas which are readily distinguishable from the 
group of plateau-basalts, and must be placed in the present series, occurs in 
the island of Mull. These rocks form part of a group of pale lavas which 
overlie the main mass of the plateau-basalts, and cap the mountain Ben 
More, together with several of its lofty neighbours. They are interstratified 
with true ophitic dolerites, and basalts showing characteristic granular augite. 
They are not so heavy as the ordinary plateau-lavas, their specific gravity 
ranging from 2 '5 5 to 2'7+. Externally they are light grey in colour and 
dull in texture, sometimes strongly amygdaloidal, sometimes with a remark- 
able platy structure, which, in the process of weathering, causes them to 
split up like stratified rocks. In some of their amygdaloidal varieties the 
cells are filled with epidote, which also appears in the fissures, and some- 
times even as a constituent of the rock. 
Specimens from this “ pale group ” of Ben More, when examined in 
thin slices under the microscope, were found by Dr. Hatch to consist almost 
wholly of felspar in minute laths or microlites, but in no instance 
sufficiently definite for satisfactory determination. In one of them he 
observed that each lath of felspar passed imperceptibly into those adjacent 
to it ; the double refraction being very weak, and the twin-striation, if 
present, not being traceable. 1 More recently my colleague, Mr. W. W. 
Watts, has looked at some of the same slides. He is disposed to class the 
1 In the course of my investigations I have had many hundreds of thin slices cut from the Ter- 
tiary volcanic rocks for microscopic determination. These I have myself studied in so far as their 
microscopic structure appeared likely to aid in the investigation of those larger questions of geologi- 
cal structure in which I was more especially interested. But for further and more detailed study I 
placed them with Dr. Hatch, who submitted to me the results of his preliminary examination, and 
