1892, 



THE SEQUENCE OF PLUTONIC ROCKS. 293 



as large as one's fist. Secondary products, after nepheline (can- 

 crinite, analcime, natrolite, &c.), are very common. For this special 

 type of rock the author proposes the term laurdalite. 



The contact-facies of the rocks of this group is represented by 

 the well-known rhomben-porphyry, and the same rock forms sheets 

 which overlie the augite-porphyrites. Boulders of this rock trans- 

 ported by the ice of the glacial period may be found on our own 

 coast near Cromer. The silica of the plutonic members of the 

 second group ranges from 51 to 58 per cent., and the alkalies from 

 9'5 or IO-5 per cent, in the laurvikites, to as much as 16 per cent, 

 in the laurdalites. 



The third group is composed of quartziferous augite-syenites. 

 These rocks have a typical granitic structure, and the felspars no 

 longer show the rhombic sections so characteristic of most of the 

 rocks belonging to the second group. They differ from the 

 laurvikites in containing plagioclase as well as orthoclase, and in the 

 abundance of brown biotite ; also in the more pronounced idiomor- 

 phism of the green pyroxene, in the absence of nepheline, sodalite, 

 and usually olivine, and in the almost constant presence of quartz. 

 They occur in much smaller masses and have a much finer grain than 

 the typical laurvikites. The author proposes the term akerite for 

 rocks having the above characters. The silica ranges from 58 to 

 62-5 per cent. 



The rocks which come next in point of time cover a large area 

 between Christiania and Lake Mjosen. They are described by the 

 author as red quartz-syenites, and the |silica ranges from 60-45 to 

 66'39 per cent. The alkali percentage is still very high (12 to 13 per 

 cent.), and the two alkalies are present in nearly equal proportions, 

 soda, however, being slightly in excess. Felspar is by far the most 

 important constituent. Orthoclase, oligoclase, and microcline are 

 present. Microperthitic intergrowths of orthoclase and albite are 

 common. The ferro-magnesian constituents include biotite, green 

 pyroxene, segirine, green hornblende, arfvedsonite, and sometimes 

 glaucophane. Titanite, zircon, apatite, and iron-ores are accessory 

 constituents. Quartz is always present, and in greater abundance 

 than in the akerites. The structure is granitic; but a special feature 

 is the occurrence of small angular drusy cavities, often lined with 

 beautiful crystals of felspar, albite, quartz, aegirine, and titanite. 

 The presence of the minerals aegirine, arfvedsonite, and glaucophane 

 is evidently connected with the high percentage of soda. These rocks, 

 like so many others from this remarkable district, are sufficiently 

 peculiar to deserve a special name, and the author accordingly 

 proposes to call them nordmarkites. 



The rocks of the fifth group -are termed soda-granites. They 

 include hornblende-granite (rich in soda) arfvedsonite-granite, and 

 segirine-granite. The silica varies from 69 to 75 per cent. The 

 dominant rock is a hornblende-granite rich in quartz. The horn- 



