294 NATURAL SCIENCE. 



JUNB. 



blende is markedly idiomorphic, and often belongs to the arfvedsonite 

 type. This type is frequently associated with a little glaucophane 

 or aegirine, Biotite is much less developed in the rocks of this 

 group than it is in those of the preceding and succeeding groups, 

 and it is frequently absent altogether. The felspar is often microcline, 

 with which albite is associated in microperthitic intergrowths. 

 Oligoclase is also present. Quartz is always present in considerable 

 abundance. The accessory minerals include titanite, zircon, orthite, 

 magnetite, and pyrite. The structure is that of a medium-grained 

 granite. The drusy or miarolitic cavities so common in nord- 

 markite are found also in the rocks of this group. 



About half-way between Lake Mjosen and the Langesundfjord 

 is an area of 700 square kilometres occupied by normal biotite-granite 

 (granitite). In all the preceding rocks soda is in excess of potash, 

 although in some cases the difference in amount of the two alkalies 

 is only slight. In the rocks of this, the sixth group, the potash is 

 in excess, and we have to deal with normal granites, such as are 

 widely distributed over the earth's surface. The silica varies from 

 75 to 77 per cent. The constituents, mentioned in the order of their 

 formation, are apatite, zircon, magnetite, titanite, biotite, felspar 

 (orthoclase, microperthite, oligoclase), and quartz. Fluorspar and 

 albite occur in drusy cavities ; and epidote and chlorite are amongst 

 the secondary products. This granite occurs in laccolitic masses, 

 and is the latest rock of the first great series of eruptions. 



The only igneous rocks which remain to be considered occur as 

 thin vertical dykes. They are extremely abundant, and as a rule 

 vary from a half to three metres in thickness, very rarely reaching 

 ten metres. They are basic in composition, and are grouped by the 

 author under the terms diabase, diabase-porphyrite, proterobase, 

 and proterobase-porphyrite. Amygdaloidal structures are common. 



Leaving these basic dykes out of account, we have in the first six 

 groups a continuous series of eruptive rocks ; that is, a series con- 

 nected by intermediate forms. The oldest basic rocks pass into the 

 laurvikites, and these into the laurdalites. Again, from the laur- 

 vikites we pass through the mica-syenites, akerites, nordmarkites, 

 and soda-granites to the normal granitites. This petrographical 

 sequence corresponds in the closest manner with the geological 

 sequence ; in other words with the order in which the rocks have 

 been formed. The oldest rocks are the most basic, and the youngest 

 rocks, excluding the dykes, are the most acid. Between the two 

 extremes we have a continuous series. The volcanic series is not 

 so complete as the plutonic, but so far as it goes the sequence is the 

 same. The fact that most of the rocks are of a peculiar type clearly 

 indicates that they must have originated in some enclosed magma- 

 basin. It is quite impossible to explain their local character and the 

 chronological sequence of the different types on the assumption that 

 the rocks are the result of the consolidation of material derived from 



