496 - TRANSACTIONS OF SECTION C, 
4. Note on the Igneous Rocks of Ordovician Age. 
By Artuur Husert Cox, M.Sc., Ph.D., F.G.S. 
The sedimentary rocks of Ordovician age consist principally of a great thick- 
ness of shales and mudstones, implying that the sediments were deposited over 
an area which, on the whole, was undergoing a slow but prolonged subsidence. 
Hence it is to be expected that the associated volcanic rocks should approximate 
to the keratophyre-spilite series. This appears to be the case with the igneous 
rocxs of most, if not all, the Ordovician areas. 
Spilites themselves have now been recorded from the Ordovician of Ayr- 
shire,” the Lleyn peninsula,’ Merionethshire,* Pembrokeshire,* Cornwall,* and 
Co. Mayo.° 
In a larger number of Ordovician volcanic areas typical spilites are absent, 
but in all cases the igneous rocks are found to bear close affinity to the rocks of 
the spilitic suite of Messrs. Dewey and Flett.* 
These authors have pointed out that ‘albitisation,’ though not confined to 
igneous rocks of the suite, is especially met with among such rocks. Now the 
British Ordovician lavas ‘are very commonly albitised, and often very completely 
so. Thus they nearly all approximate to the keratophyres; some of them— 
namely, the ‘soda rhyolites "—being very acid, are quartz-keratophyres, accord- 
ing to Rosenbusch’s nomenclature; others, the ‘andesites,’ of intermediate to 
almost basic character, are the keratophyres proper; while the ‘acid andesites’ 
and ‘ albite-trachytes ** would also be denoted as keratophyres on the Continent. 
In many areas every transition exists between the acid and intermediate rocks, 
while less commonly the more basic representatives of this series, the spilites 
proper, are to be found. 
Although the Ordovician igneous rocks mostly have a high soda percentage, 
soda-amphiboles and soda-pyroxenes seem almost entirely absent. Riebeckite 
occurs in the Mynydd Mawr rock,’ and perhaps in a few of the rhyolites,* while 
the peculiar pleochroism of certain hornblendes occurring as accessory con- 
stituents in some of the diabases probably points to the presence of soda in the 
molecule. There are, however, very few such cases recorded, considering the 
large number of rocks described, even if due allowance is made for many of the 
descriptions being by no means recent. Felspathoid minerals are also entirely 
absent from these Ordovician rocks. ‘ 
This absence of felspathoid minerals and of soda-hornblendes and soda-augites 
constitutes an important difference between these rocks and those commonly 
described as alkaline or Atlantic in type. It may be neted that Rosenbusch® 
first placed the keratophyres among his alkaline division, but later, taking 
into consideration the associated types, classed them with the calcalkaime 
rocks. 
A point worth notice in the Ordovician igneous rock is the frequent presence 
of a rhombic pyroxene. Rosenbusch has pointed out that, whereas in calc- 
alkaline (Pacific) rocks olivine makes its appearance at an early stage as the 
rocks become basic, in the alkaline (Atlantic) rocks there is a marked tendency 
for the réle of olivine to be taken by hornblende and biotite, even in some cases 
where the magma was ultrabasic. 
In the keratophyres rhombic pyroxenes are of very common occurrence, and 
1H. Dewey and J. 8. Flett, Geol. Mag., 1911, p. 202. 
2 The Silurian Rocks of Britain, vol. i., Scotland, Mem, Geol. Surv., 1899, 
p. 85. 
3 A. H. Cox and O. T. Jones, this report. 
4 H. Fox and J. J. H. Teall, Quart. Journ. Geol. Soc., vol. 49 (1893), p. 85; 
and Mem. Geol. Surv., ‘The Geology of the Lizard,’ 1912, p. 165. 
5G, J. Gardiner and §. H. Reynolds, Quart. Journ. Geol. Soc., vol. 65 
(1909), p. 136; and vol. 68 (1912), p. 75. 
* H. H. Thomas, Quart. Journ. Geol. Soc., vol. 67 (1911), p. 193. 
1 A, Harker, Bala Volcanic Series, 1889, p. 50. 
8 W. G. Fearnsides, Proc. Geol. Assoc., vol. 22 (1912), p. 206. 
® Mikroskopische Physiegraphie der Massigen Gesteine, vol. 2, pt. 2 (1908) , 
p. 1493. 
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