part 4] NORiTE OF sierra leoi^e. 303 



b}'" gravity, while others show a similar arrangement due to succes- 

 sive intrusions, as exemplified by the basic and ultrabasic Tertiary 

 igneous rocks of Rum.^ 



Other norites, such as those associated with the Gallowa}^ masses,^ 

 occur as basic phases of granitic intrusions ; while yet others, 

 such as those of St. David's Head,^ form one member of a series 

 that comprises a wide range of rock-types. The norite which we 

 are now considering possibly possessed a marginal phase, long since 

 swept away ; but it never had any stratiform arrangement, unless 

 it was originally very extensive as compared with its present 

 size. The following observations on the specific gravity of the 

 norite, ranging from sea-level to the summit of the highest 

 peak (an altitude of nearly 3000 feet), indicate great irregularity 

 of structure. 



Approximate o -^ 



Localitu. elevation, ^ ., 



'' ■ J- , firavity. 



m feet. '' ^ 



1. Landing-- stag-e, Murray Town 3'06 



2. Banana Islands 3'1 7 



3. Foot of Charlotte Falls 400 3-23 



4. Sand Elevator 500 2-99 



5. Wilberforce 700 2-95 



6. Waterloo-York road 1000 3-26 



7. Leicester Peak 1400 2-87 



8. Leicester Peak 1950 2-98 



9. PicketHill 2900 2'80 



Average of twelve specimens 3*01 



Numerous observations on mineralogical composition yield a 

 similar result. Nevertheless, in the variety and sometimes the 

 succession of its associated minor intrusions, the norite bears a 

 strong resemblance to certain other basic masses, such as the 

 norites of St. David's Head, the Duluth laccolite,^ and the gabbro 

 of the CuiUin Hills.5 



(4j The Intrusions. 



The stock is thus made up essentially of a noritic intrusion of 

 fairly uniform character. But the original mass was invaded 

 in quick succession by several cognate sub-magmas, all of which 

 were, however, relativel}^ of very small volume ; at a later date the 

 complex was cut by numerous dykes of enstatite-dolerite. Most 

 of these rocks are well exposed along the south-western foreshore 



1 'Geology of the Small Isles of Inverness' Mem. Geol. Surv. Scot. 1908, 

 p. 68. 



2 ' The Silurian Eocks of Scotland' Mem. Geol. Surv. 1899, p. 613. 



3 J. y. Elsden, ' The St. David's Head Rock Series ' Q. J. G. S. vol. Ixiv 

 (1908) p. 273. 



■* M. L. Nebel, ' The Basal Phases of the Duluth Gabbro near Gabamichi- 

 gami Lake (Minnesota), & its Contact- Effects ' Econ. Geol. vol. xiv (1919) 

 p. 367. See also N. H. Winchell on this intrusion in Final Eep. Geol. Surv. 

 Minnesota, vols, iv & v (1899-1900). 



•^ ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surv. 1904, p. 82. 



