756 H. A. BROUWER 



in detail. Because the study of the alkali syenites and effusive 

 rocks which were collected during my visit in 1910 has been post- 

 poned by my departure for the East Indies, we will mention the 

 most characteristic features of the latter rocks, as they have been 

 briefly described by Humphrey. 



The richness in pneumatolytic and thermal minerals (especially 

 fluorspar) is characteristic for the rocks of the whole region ; accord- 

 ingly the loss on ignition is always considerable. 



Besides the coarse- to medium-grained intrusive rocks and the 

 effusive ones, transitional types have a great development. A 

 series of porphyrites which graduate through all the types between 

 true lavas, intrusive sheets, and dikes often form an unbroken 

 series paralleling the effusive rocks. 



Nepheline syenites : The nepheline syenites partly belong to the 

 group of the foyaites and are usually coarse-grained. They are 

 connected by transitions with the lujaurites, which are characterized 

 by the abundance of small needle-shaped crystals of aegirine and 

 are quite similar to the lujaurites of Greenland and the peninsula 

 of Kola. The latter rocks principally are found in the southern 

 part of the mountains, while lujauritic rocks only exceptionally 

 occur in the central part. The foyaites have been frequently found 

 by Humphrey in dikes, traversing the various effusive rocks and the 

 red syenites. 



If we divide the foyaites according to the character of the dark 

 minerals, we find that nearly all the subgroups are represented and, 

 in the porphyrftic varieties, we also find aegirine, alkaline amphi- 

 boles, and biotite, either characterizing different rock types or 

 occurring together in the same rock. The amphiboles are rich in 

 alkalies and often show peculiar properties which are similar to 

 those often mentioned in the literature of the foyaitic and theralitic 

 rocks. Their optical and chemical properties have not yet been 

 studied in detail, but they are known to have the properties of 

 the barkevikitic, kataforitic, or arfvedsonitic amphiboles. 



The isolated range of hills to the southeast of the Pilandsberg 

 proper, which bends around with the lujaurites, consists of aegirine- 

 amphibole foyaites, in which the amphibole has a pronounced zonal 

 structure. The differences in color are progressive from brown 



