26 



BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON 



[Memoirs National 

 [Vol. XIX, 



According to the views of Franchi ('06) and Schmidt ('08), the Ivrea zone of basic intrusive 

 rocks should be classed among the Permo-Carboniferous formations, while Suess considers it of 

 common origin with the green-rocks of the Piedmontese Alps. The mass is infolded in the mica- 

 schists without evidence of transgressive intrusion, and contains a group of rocks ranging from 

 granite, quartz, diorite, gabbro and norite to peridotite, but diabase is absent. There is a fairly 

 regular arrangement of the rock-types, the highly basic rocks collecting toward the west, while 

 in the east the diorites predominate. Suess (IV, pp. 134, 566) would correlate this zone with 

 the tonalite series of Monzoni farther to the east, and believes it to have been injected into the 

 sole-plane upon which the Dinarides were thrust on to the Alps in which case the western would 

 be the lower side of the plutonic complex. According to Spitz ('15) these intrusions are in par*- 

 Tertiary, in part pre-Permian, and there are some portions of uncertain age. Kober ('21) holds 

 that they are partly of late Mesozoic age, and partly younger than the folding movements 

 "But the problem of the Dinaric cicatrice with which the Alpine geologists have been so long 

 concerned is not yet completely solved." 



There are very many details concerning these much-discussed rocks for which reference 

 must be made to the authorities cited by Suess (IV, p. 130-134) or to later discussions, e. g. that 

 of Kober ('21) which propound different explanations of the Ivrea zone. 



>a5 



hrea Strona 



1. Gneiss. 2. Granite. 



Fig. 7. — Geological section across the Pennine Alps. (After Argand.) 

 3. Basic plutonic rocks. 4. Alpine Triassic and Jurassic rocks. 5. Southern Permian and Triassic rocks. 



THE SWISS ALPS. 



In regard to the Lepontine Alps there are again to be noted the divergences of opinion 

 between the Italian authors and the Swiss geologists, whose views are accepted in the main by 

 Suess. The former are disposed to consider the bulk of the mountains as autochthonous over- 

 thrust folds, but to the Swiss geologists, following the researches of Lugeon they appear to 

 be made up of a series of immense largely exotic recumbent folds. The great band of basic 

 and ultrabasic rocks which extends from Greisoney and the Breithorn towards Zermatt, as also 

 those in the lower parts of the Dente Blanche and the Matterhorn are correlated by Argand 

 ('06) with the rocks of Ivrea (see fig. 7), a correlation to which objection has been raised by 

 Novarese ('06) on petrographic grounds, particularly in the absence of diabase from the Ivrea 

 zone. Schmidt ('07, '08), on the other hand (see fig. 8), considers the Ivrea basic rocks as older 

 than those of the Dente Blanche, probably Carboniferous, and believes all the Dente Blanche 

 group of green-rocks, and the calc-schists in which they occur, to be derived from the Dinaric 

 Alps, and thrust through the syncline between Mont Blanc and St. Gotthard. He classes, also, 

 as an extension of the same flake, the upper portion of the Freiberg Alps in which basic rocks 

 were found by Lugeon ('96) and Jaccard ('04), with which are some radiolarian cherts. 



Beneath the great mass of basic plutonic rocks of the Dente Blanche sheet lie the Bundner- 

 schiefer corresponding to the Mesozoic calc-schists, containing other green-rocks, which are 

 apparently a series of intercalated contemporaneous flows and tuffs with shallow intrusions. 

 In the Bagnetal, Woyno ('11) showed the green-schists, glaucophane schists, prasinites, and 

 serpentine were probably derived from slightly sodic tuffs and lavas. 5 Further to the northeast 

 near Brig the green-rocks have been studied by Schmidt ('08) and Preiswerk ('03, '07) . The Mes- 



6 The writer examined specimens of these rocks in Basel and Zurich through the court esy of Drs. Preiswerk and Woyno. They are very different 

 from the rocks of the Ivrea zone studied by Schaefer ( '9S) and those forming the gneissic gabbros and allaiinites of Zermatt and the Saasthal (Schaefer 

 '95 Bonney '92) of which representative collections were examined by the writer in Heidelberg and Cambridge. 



