CHAP. XVII 
MATERIALS ERUPTED— DIABASE-LAVAS 
273 
ii. CHARACTEKS OF THE MATERIALS ERUPTED BY THE VOLCANOES 
A general summary of the petrographical diaracters of the igneous rocks 
of the 'lower Old Eed Sandstone may here find a place. Further details 
will be given in the account of “ Lake Caledonia,” which is the typical area 
for thein ; but, on the whole, the prevailing types in one region are found 
to be repeated in the others. 
1 . Bedded Zaras.— Beginning with the lavas which were poured out at 
the surface, we have to notice a considerable range of chemical composition 
among them, although, as a rule, they are characterized by general similarity 
of external appearance. At the one end, come diabases and other ancient 
forms of basalt or dolerite, wherein the silica percentage is below or little 
above 50 . By far the largest proportion of the lavas, however, are por- 
iihyrites or altered andesites, having about GO per cent ot silica. With these 
are associated lavas containing more or less unstriped felspar and a some- 
what higher proportion of silica, which may be grouped as trachytes, 
thoiio-h 110 very sharp line can be drawn between them and the andesites. In 
the Pentland Hills, and some other areas, orthophyres flowed out alternately 
with the more basic lavas, and were associated with felsitic tuffs and 
brcccifts* 
It is noteworthy that the lava-sheets of the Lower Old Eed Sandstone, 
if we consider the character of the prevalent type, hold an intermediate 
grade between the average chemical composition ot those of Silurian and ot 
those of later Carboniferous time. On the one hand, they rare y assume 
the character of- thoroughly acid rocks, like the nodular rhyolites ot the Ba a 
and Upper Silurian series on the other hand, they seldom include such 
basic lavas as the basalts, so common among the puy-eruptions ot the 
Carboniferous system, and never, so far as I know, contain varieties com- 
parable to the “ultra-basic” compounds which I shall have occasion to 
allude to as characteristic of a particular volcanic zone ui that system. 
(a) The TJiabase-lavas are typically developed 111 the chain ot the 
Pentland Hills, where they form long bands intercalated between felsitic 
tuffs — a remarkable association, to which I shall make more detailed 
reference in a later chapter. They range in texture from a compact dark 
greenish base to a dull earthy amygdaloid. One of their most remark- 
able varieties is a fine-grained green porphyry, with large fiat tabulai & 
of plagioclase arranged parallel to the direction o flow (Carnethy Hill). 
Most of them, however, are more or less amygdaloidal, and some of them 
(Warklaw Hill) strongly so. The following analyses, made in the Ldioratorj 
of the Eoyal School of Mines under the direction ot 1 1’of- L Irankland 
show the chemical composition of some of the diabases of the lentland 
Hills : 
' The only examples known to me are those of Benann More and other 
^ For analyses of some Shetland diabases of Old Red Sandstone age, see Mi. R. R. Tatlock, 
Trans. Roy. Soc. Edin. vol. xxxii. (1887), p. 387. ^ 
VOL. I 
