Chapter III. 



THE BASIC AND ULTRABASIC INTRUSIVE ROCKS IN NORTHERN 



EUROPE. 



THE BRITISH ISLES. 



Probably tbere is no country of similar area witb a more lengtby and complex record of 

 igneous activity tban tbat of tbo British Isles, or one which has been studied more intensively 

 from the tectonico-petrographic standpoint. The facts have recently been summarized in 

 Harker's comprehensive address ('17), to which the writer is much indebted, and also to Sir 

 A. Geikie's "Ancient Volcanoes of Great Britain" ('97). We here consider very briefly those 

 portions only which bear upon the basic rocks. 



Pre-Camhrian. — According to the views of Barrow ('12) the oldest formation of the Scotch 

 Highland, the Dalradian series, though very highly metamorphosed, indicates the association of a 

 series of lava flows, in part pillow-lavas, with intrusive contemporaneous sills of dolerite, albite- 

 dolerite, keratophyre, and soda granite, interstratified in a series of quartzites, shales and lime- 

 stones. In Aberdeenshire they are now mostly amphibolites, but in Banffshire and Argyllshire 

 their affinities with the spilitic rocks are indicated (Peach '04, '09). Their development was 

 followed by orogenic movements and igneous intrusions which produced the majority of the 

 gneissic and igneous rocks of the Scotch Highlands, the older igneous rocks of the Highlands. 

 The earliest of these were of stratiform masses of hypersthene-gabbros, and occasionally 

 pyroxenite or peridotites in narrow sills. 1 All of these suffered intense folding and were 

 subsequently invaded by granitic gneisses. Barrow ('12) considers that the great period of 

 folding separates these two series of intrusions, and that the former basic intrusions were injected 

 into the Highland sediments and lavas before they had been much disturbed. Harker ('17), 

 on the other hand, thinks that the intrusion of the basic rocks accompanied the main folding in 

 which their sill-form was determined, while the granitic masses of more irregular outline formed 

 during the times which directly followed the main period of orogenic movement. (See also Read 

 '19 and fig. 1.) More clearly laccolitic types of igneous complex occur occasionally, such as at 

 Carn Chinneag, where the granite has a differentiated margin of diorite and gabbroid rocks. 

 Here the division of the plutonic action into two sharply defined phases is not apparent. Similar 

 pre-Cambrian albitic pillow-lavas occur in the Lleyn Peninsula and Bardsey Island of North 

 Wales and in Anglesey, where their eruption seems to have occurred under exactly the condi- 

 tions specified for the development of the "spilitic suite." The ancient sediments and inter- 

 calated volcanic rocks were subsequently intensely folded and invaded by a series of probably 

 late pre-Cambrian plutonic rocks ranging from peridotite, mostly dunite, to a rather sodic 

 granite, the latest of the series. (Greenley '19.) 



The Lizard region is different from these. The oldest series of rocks, the Lizard Head schists 

 and granulites were interstratified with basic sills and lava-flows and ashes. They were invaded 

 by tonalitic rocks and cut by dolerite-dikes. Great crust-movements followed, after which 

 were injected coarse dolerites or gabbros (now hornblendites) , which were immediately followed 

 by the intrusion of a roughly circular, probably laccolitic mass of peridotite, within which is 

 lherzolite andharzbergite. Fluxion-banding is a marked feature of these rocks. "The zoned 

 structure is clear evidence that the serpentine is an intrusive stock that welled upward and 

 forced outward the surrounding schists. The fine 'flinty-looking' serpentine of the margin was 

 the earliest, and the coarse bastite-serpentine the latest portion of the intrusion." (Flett and 



1 The ultrabasic rocks of Comiemarra probably belong to this series, but little is known definitely about them. 



