CHAP. XXIV 
materials of the plateau-eruptions 
377 
Tlie same two plateaux likewise si^pply further illustrations of the out- 
flow of similar volcanic materials in the same locality at widely separated 
intervals of time. They may be traced up to and round the margin of 
the great pile of andesites of Lower Old lied Sandstone age forming the 
Cheviot Hills. 
ii. NATURE OF THE MATERIALS ERUPTED 
The volcanic materials characteristic of the plateau-type ot eruptions 
consist mainly of lavas in successive sheets, but include also various tuffs 
in frequent thin courses, and less commonly in thick local accumulations. 
The lavas are chiefly andesites in the altered condition of porphyrites. 
They vary a good deal in the relative proportions of silica. Some of tlieni 
are decidedly basic and take the form of dolerites and olivine-basalts. With 
these rocks are occasionally associated “ultra-basic” varieties, where the 
felspar almost disappears and the material consists mainly of ferro-magnesian 
minerals. The more basic rocks are generally found towards the bottom of 
the volcanic series, where they appear as the oldest flows. In the Garleton 
Hills lavas of a much more acid nature are met with — true sanidine- 
trachytes, which overlie the porphyrites and basalts of the earlier eruptions. 
No adequate investigation has yet been made of the chemical and micro- 
scopic characters of these various rocks, regarded as a great volcanic series 
belonging to a definite geological age, though many of the individual rocks 
and the petrography of different districts have been more or less full} 
described. I cannot here enter into much detail on the subject, but must 
content myself with such a summary as will convey some idea of the general 
composition and structure of this very interesting volcanic series. 
(«) Augite-oltvine Hocks (Pigrites and Limburgites). — Towards the 
bottom of the plateaux there are found here and there sheets of “ ultra-basic 
material, some of which appear to be bedded with the other rocks and to 
have flowed out as surface-lavas, though it may be impossible to prove that 
they are not sills. Thus at AVhitelaw Hill, on the south side of the Garleton 
Hills, a dark heavy rock is found to contain hardly any felspar, but to be 
made up mainly of olivine and augite. Dr. Hatch has published a descrip- 
tion and drawing of this rock, together with the following analysis by Mr. 
Player : ^ — 
Silica 
Titanic oxide 
Alumina . 
Ferric oxide 
Ferrou.s oxide 
Lime 
iMagnesia . 
Potash 
Soda 
Loss by ignition 
40-2 
2'9 
12-8 
4-0 
10-4 
10- 4 
11- 9 
0-8 
2- 7 
3- 4 
Spec. grav. 3-03. 
1 Trmis. Hoy. Soc. Edin. vol. xxxvii. (1893), p. 116. 
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