Academt of Sciences] NORTHERN EUROPE. 11 



No. 1] 



these plutonic rocks can be inferred from the occurrence of pebbles of serpentine andgabbroin 

 the Silurian rocks. The same zone of Ordovician sediments with intercalated basic rocks may 

 be traced into Ireland, where pillow-lavas occur with chert in Tyrone (Geikie '97), Mayo, and 

 Galway (Reynolds and Gardiner '09, '10, '14). The sediments and igneous rocks are here of 

 Bala age, and were thus formed at a period nearer to the epoch of folding than the Arenig rocks. 

 The occurrence of conglomerates among the sediments, and lamprophyre among the intrusive 

 rocks, are features not recognized in Flett's conception of a typical development of the 

 spilitic suite. Another instance of the association of somewhat diverse types of igneous rocks 

 lies in the interdigitation of normal basaltic lavas with those of a spditic facies rich in acid 

 plagioclase, skomerites, etc., in south Wales— Skomer, Haverfordwest, Llangynog, and Fish- 

 guard. These are variously of Arenig, Llanvirn, and Llandeilo age. (Thomas and Cantrill '06, 

 Thomas '11, '14, Cox and Jones '13, Cox '15.) On the other hand, the Ordovician greenstones 

 of Cornwall, like those of Scotland, are thoroughly representative of the spilitic petrograph- 

 ical characters, and their occurrence affords the typical examples of the association of these 

 with radiolarian sediment, as Dewey and Flett ('11) and others have shown. 



In north Wales, the Ordovician rocks are usually of Llandeilo age, and are generally free 

 from any alkaline features. There occur several sill-like masses of basic rocks, some of which 

 are so constantly associated with the arches or troughs of folds as to be probably phacolites 

 rather than laccolites, and to have been injected during the late Ordovician folding. The most 

 interesting is that of Mynyth Penarfynydd, which is perhaps in part gravitationally differen- 

 tiated, the mass of hornblende-diabase having a thick layer of olivine-dolerite near its base. 

 Although there is no very gradual transition between the two rocks they are evidently in close 

 connection, and indeed have segregation-veins passing from one to the other. (Harker '89.) The 

 minor intrusions which invaded the older Paleozoic rocks of Anglesey during the late Silurian 

 folding, though partly felsites, have for their latest members dikes of dolerite and sills of horn- 

 blende-picrite, which, though affected by the later stages of the orogeny, lie concordantly within 

 the cleavage but not the bedding planes of the invaded formations. (Greenley '19.) 



Completely different from these are the rocks described by Shand ('10) at Loch Borolan in 

 northwest Scotland, though they are possibly of the same age. Here are a very remarkable 

 series of garnetiferous ultrabasic rocks associated with alkaline types. In places the differ- 

 entiation appears due to gravitational separation; in other places sharp lines of division appear 

 between the rock types. The relationship is obscured by imperfect exposure and crushing 

 subsequent to consolidation and cooling, and it is not clear what tectonic conditions accompa- 

 nied their intrusion. 



The middle and late Paleozoic intrusions. — The folding which followed Silurian times was 

 accompanied by the intrusion of the newer granites of Scotland which with one exception are 

 entirely subalkaline. In occasional occurrences as at Garabal Hill (Teall and Daykins '92, 

 Wylie and Scott '13), at Glen Doll, and the Coyles (Barrow '12), basic and ultrabasic rocks 

 are developed. In the first of these a rock, which is a hybrid of acid and basic types, has been 

 noted by Wylie and Scott, and in the Glen Doll complex a hornblendic rock occurs of probably 

 similar nature. 3 Generally, however, the successive intrusions are sharply denned. The bound- 

 aries of the ultrabasic masses follow the structure lines of the invaded formation much more 

 strongly than do those of the acid types, which are generally transgressive. The kentellanites 

 (olivine-monzonites) of Argyllshire, though of a somewhat alkaline nature, have a fairly strati- 

 form habit, and exhibit a distinct gravitiatonal separation. They are associated with an augite- 

 diorite marginal facies of the granite boss of Ben Cruachan (Hill and Kynaston '00, '08). Fur- 

 ther kentellanites and still more basic cortlandites near Ballachulish are similarly related to the 

 granites of Ben Nevis (Bailey and Maufe '16), and quartz-syenites, monzonite, "picrite," and 

 hornblendite occur together on Colonsay (Bailey and Wright '11). The basic plutonic rocks of 

 Huntly in Aberdeenshire, described by Watt ('14), perhaps afford a further instance of a 



3 The writer had the privilege of seeing this area with Dr. Harker and>Mr. Barrow. 

 81795°— 26 2 



