396 MICA-GNEISS AND HORNBLENDIC GNEISS. 



gneiss from Freiburg in Saxony. In another part of the Humboldt 

 range, the texture of the gneiss approaches more nearly to granite ; 

 and in the Secret Pass the rock contains crystals which resemble 

 zircon, though no zirconia has been detected on analysis. 



Hornblende gneisses generally predominate in the Ogden and 

 Farmington Canons in the Wahsatch range, Utah. One type is made 

 up of orthoclase, with a good deal of plagioclase, quartz, brown mica, 

 much hornblende, and apatite. The quantity of plagioclase is in some 

 cases much greater. At other times a mineral like zircon is present, 

 and in some of these the quantity of hornblende is small. Here too 

 some of the hornblende gneiss contains garnet. 



In the same district mica gneiss is found, which is free from 

 hornblende, is rich in felspar and quartz, and poor in brown mica and 

 deep red garnet. 



Zirkel contrasts the mica gneiss with the hornblende gneiss, showing 

 that in the former orthoclase preponderates ; there is little plagioclase ; 

 the quartz is full of fluid inclusions ; apatite and zircon are rare 

 or absent, and titanite is entirely absent. In hornblende gneiss 

 plagioclase sometimes preponderates. Fluid inclusions in quartz are 

 comparatively rare. Apatite and zircon are usually abundant, and 

 titanite is sometimes present. 1 



Succession of North American Crystalline Rocks. The lowest 

 group of the Laurentian Rocks of North America is named the 

 Ottawa series. It consists of granitic gneisses, red and grey, often 

 highly contorted, formed of orthoclase, with quartz and hornblende. 

 Next in succession is the Grenville series, quartzose gneisses, and 

 quartzites, with great beds of Dolomite, and the Limestone which 

 yields Eozoon. It is said to be 17,000 feet thick. The Norian or 

 Labrador series is supposed to rest unconformably on the older 

 gneisses. These gneisses contain Labradorite or Anorthite, are 10,000 

 feet thick, and spread over Labrador and north of the St. Lawrence. 

 The Huronian series is a yet newer group of metamorphic rocks, con- 

 sisting, according to Dr. Bigsby, of chloritic, siliceous, and hornblende 

 slates ; but it also contains limestones, serpentines, and rocks of a 

 greenstone character. It has been compared to the Pebidian rocks of 

 Wales. The Montalban series is a succession of micaceous schists and 

 friable grey gneisses, passing into micaceous quartzites and hornblendic 

 gneiss. Newer still is the Taconian series, which forms the Taconic 

 hills. It consists of quartz rocks and schists, with crystalline magnesian 

 limestone, some serpentine, and great deposits of iron ores. Attempts 

 have been made to parallel these rocks in Europe, but the method of 

 research has yet to be developed which would justify the correlation. 2 



1 Zirkel, Micros. Petrog. 



2 See Hicks : " Succession of the Archaean Rocks of America, compared with 

 the Pre-Cambriau Rocks of Europe," Proc. Geol. Assoc., vol. viii. No. 5. 



