Academy of Sciences] 

 No. 1] 



NORTHERN EUROPE. 



13 



objections to this process, and holds that the basic marginal facies of laccolitic complexes are- 

 the portions of the magma that were cooled and crystallized quickly with but little differentia- 

 tion. At Carrock Fell these marginal rocks about the central mass of quartz-gabbro contain 

 as much as 22 per cent of iron ores, so that if this explanation be true, the original magma 

 must either have been very abnormal, of which there is not other evidence, or a considerable 

 se°re°-ation of iron-ore to both the floor and roof of the sheet of laccolite must have occurred, as 

 Daly recognizes ('14 p. 359). 



The Tertiary intrusions. — No further development of ultrabasic or basic plutonic rocks 

 occurred till Tertiary times, when igneous activity again centered in the northwestern areas. 

 Sir A. Geikie ('97) and Harker have shown that the plateau basalts of the Brito-Icelandic 

 petrographic province as recognized first by Judd ('86) were formed by a series of fissure eruptions 

 of which merely the horsts remain above sea level. They have a slightly alkaline facies, indi- 

 cated by the occasional abundance of sodic zeolites (Harker '11), and the presence of definitely 

 alkaline rocks, such as the nepheline-rock of Shiant, the ouachitite of Mull, and the rockallite 

 of Rockall. About centers of tangential pressure (from which the abundant dikes tend to 

 radiate), such as the Cuillins in Skye, St. Kilda, Eigg, Rum, Muck, Mull, Ardnamurchan, 

 Arran, and Carlingford, there appear laccolitic complexes of peridotite and gabbro invading 

 the basalts and invaded by granite. That of Skye (Fig. 3) has become classic through the 

 investgiations of Harker ('04). 

 Only a brief summary can be given 

 of the varied features of this com- 

 plex. The overlapping of the older 

 formations around it shows that 

 it was long a center of elevation. 

 The volcanic phase was followed 

 by the intrusion through fissures of 

 varied peridotitic rocks, in alternat- 

 ing bands or in masses in whieh a 

 later rock veins an earlier one, or 

 is crowded with debris of it They 

 were succeeded by much larger in- 

 trusions of gabbro, the western por- 

 tion being laccolitic, and the east- 

 ern boss-like, the latter being the 

 region of most narrowly localized 

 and intense strain. The gabbro- 

 laccolite was built up of a multitude of distinct intrusions which differ somewhat in composition 

 and structure, and often visibly cut one another. In some places no interval of quiescence 

 separated the period of intrusion of the gabbro from that of the succeeding granite, as is shown 

 by their extreme interaction, but in one place the granite has been so chilled against the gabbro 

 that its margin and apophyses have assumed the structure of spherulitic rhyolite. The granite 

 has also assumed the laccolitic habit in the west and the boss-form in the east. In the former 

 it was intruded partly beneath and into the gabbro. The granite-boss rises from the dome of 

 an old anticline which has been a center of uplift since Paleozoic times, and the upward thrust 

 was renewed after the injection of the granite. The laccolitic gabbro-granite mass, on the 

 other hand, has a synclinal structure, due perhaps to subsidence into the region from which 

 the magma was extravasated. A series of minor intrusions were formed after this, dolerite- 

 sills in the plateau-basalts, dolerite-sheets in the gabbro, and a few ultra-basic dikes near thereto, 

 while composite sills and granophyres formed about the boss of granite. The complexity of 

 these relations are held by Harker ('16) completely to exclude the possibility that the complex 

 could have formed by gravitative differentiation of a laccolite with a general easterly dip 

 formed as the result of "a single principal act of intrusion of magma" as suggested by Bowen 

 ('15). 



Fig. 2.— Geological map of Carrock fell, Cumberland. (Alter Harker.) 



A. Gabbro increasingly basic toward margin. 



B. Granopbyre rendered basic on margin by absorption of Gabbro. C. Greisen. 

 Invaded formations: Slates, sbales, and old basalts. 



