THE CLASSIFICATION OF IGNEOUS ROCKS. 483 



common, while in other characters they present a continuous 

 variation from one end of the series to the other. One such 

 series, comprising the types grorudite, solvsbergite, and 

 tino-uaite, is described in detail in the first instalment of the 

 author's " Eruptivgesteine des Kristianiagebietes". These 

 are hypabyssal rocks occurring as dykes. They have 

 in common the constantly low percentages of lime and 

 magnesia, the preponderance of soda over potash, etc. ; 

 while the proportions of other constituents vary consider- 

 ably, though in a regular manner, the percentage of silica 

 falling from seventy-four to fifty-six, that of alumina rising 

 from eight to twenty, and that of the alkalies jointly from 

 eight to sixteen. 



The types just mentioned belong to different families, 

 which are also represented in the district both by plutonic 

 and by other hypabyssal series. Thus the grorudite 

 belongs to the family (or rather subfamily) of soda-granite. 

 It agrees with this plutonic rock somewhat closely in 

 chemical composition, but is rather richer in ferric oxide 

 and alumina. It is therefore to be referred to the " dia- 

 schiste," but does not imply an extreme differentiation as 

 compared with the soda-granite. Solvsbergite bears a like 

 relation to the quartz-syenite of the district (nordmarkite), 

 and tingaaite to the nepheline-syenite (laurdalite). The 

 three dyke-rocks thus seem to have had similar histories 

 and to play corresponding parts in the great group of 

 associated igneous rocks of the district. Another series 

 includes the rhomb-porphyries, some of which are well- 

 known rocks. These have special characters in common, 

 such as the peculiar crystallographic habit of the felspar 

 crystals, which distinguishes them at once from the other 

 hypabyssal series ; but they embrace a considerable range 

 of types belonging to different families. The plagioclase- 

 rhomb-porphyry corresponds to the plutonic gabbro-diabase, 

 the ordinary type to the augite-syenite (laurvikite), the 

 nepheline-bearing type to the laurdalite, and the quartz- 

 bearing to the nordmarkite. 



Contrasted with the idea of a rock series is what the 

 Norwegian geologist terms a " Faciessuite," comprising a 



34 



