TRANSACTIONS OF SECTION C. 497 
are sometimes the only dark minerals present. In the spilites and many related 
basalts a mineral of closely related composition, ‘magnesium-diopoide,’ plays an 
important part. The same mineral, also described as ‘ enstatite-augite,’ *° 
‘sahlite,’ &c., and often regarded as an intergrowth of rhombic and monoclinic 
pyroxenes,’! is very common among the diabases found in association with 
spilites and keratophyres, and the diabases may often be quite basic without 
containing any olivine.’ 
Thus in rocks of this suite rhombic pyroxenes (in some cases intergrown with 
monoclinic pyroxenes) would appear to take in some degree the place of olivine in 
the corresponding Pacific rocks, and of hornblende (and biotite) in corresponding 
Atlantic types. 
A striking feature in the various volcanic series of Ordovician age is the 
importance of the pyroclastic rocks. It is not uncommon to find tuffs of 
various kinds build up the whole, or nearly so, of a thick volcanic series. This 
is in marked contrast to all the volcanic series of later date in the British area, 
and is not entirely explained by the fact that the eruptions were submarine, and 
that therefore the elastic products were more likely to be preserved. The further 
explanation is probably to be looked for in the viscosity of the magmas. This 
high viscosity has been noted in the spilites and some other rocks,’* and is 
apparent from the field relations in most cases. It is just such highly viscous 
lavas that one would expect to furnish a large amount of pyroclastic material.“ 
In this connection it may be pointed out that, whereas the more basic rocks of 
the keratophyre-spilite series are almost invariably highly vesicular, the acid 
and acid-intermediate members of the series (quartz-keratophyres, &c.) are as 
frequently non-vesicular. 
It has been shown by Messrs. Dewey and Flett ** that rocks of the spilitic 
suite are found in areas which have undergone a slow but prolonged subsidence ; 
hence such rocks will be typically produced in areas in which a geosyncline is 
in process of formation. Now from the theory of isostasy we may expect such 
areas to be soon affected by a movement of compression resulting in mountain- 
building, with accompanying folding, cleavage, and thrusting. This type of 
movement will very likely be closely associated with the production of rocks of a 
calcalkaline (Pacific) type. 
In short, areas in which spilitic rocks are found will usually be strongly 
affected by cleavage, and will often be complicated by the occurrence of igneous 
rocks of a later and (apparently) quite different cycle. 
In Britain good examples of this sequence of events are seen in the relation 
of (i.) the post-Silurian calcalkaline igneous rocks of Scotland and Ireland to the 
spilitic lavas of Ordovician ** age, and (ii.) the igneous rocks connected with the 
uprise of the Armorican chain to the spilitic rocks of Middle and Upper 
Devonian age in Devon and Cornwall. The occurrence of spilitic rocks in areas 
which have been subjected to overfolding was noted by Steinman.” 
The Ordovician igneous rocks would appear to afford a favourable ground for 
ascertaining whether the connection between rock-types and types of earth- 
movement holds good to a greater extent than has been hitherto suggested. The 
great succession of Ordovician shales and mudstones is locally interrupted by 
coarser deposits, or even by small unconformities implying a certain amount of 
uplift, or, at any rate, a checking in the rate of subsidence. Further, the sub- 
sidence continued in certain areas into Silurian time, while in other localities 
there is a more or less strong unconformity between the two systems. 
10 Wahl, Tscherm, Min. und Petr, Mitth, n. s., vol. 26 (1907), pp. 21 and 26. 
1 J. V. Elsden, Quart. Journ. Geol. Soc., vol. 64 (1908), p. 287, with 
references. 
12 Harker, Bala Volcanic Series (1889), p. 77. 
13 H. H. Thomas, Quart. Journ. Geol. Soc., vol. 64 (1911), p. 195. 
14 See Dakyns and Greenly, ‘The Felsitic Slates of Snowdon,’ Geol. Mag., 
1905, p. 541. 
% Op. supra cit. 
16 The few examples of contemporaneous igneous rocks in the Silurian appear 
to be keratophyres and spilites (excluding the Downtonian lavas of Kincardine- 
shire). 
1” Ber, Nat. Gesell. Freiburg i. B., 16 (1905), p. 44. 
1913. K K 
