Chapter IV. 



THE BASIC AND ULTRABASIC INTRUSIVE ROCKS OF THE ALPINE 

 MOUNTAIN SYSTEM IN SOUTHERN EUROPE AND THE ATLAS 

 MOUNTAINS. 



The younger basic intrusions of the Pyrenees occur sometimes in rounded masses, at other 

 times as parts of dikes, and again as intercalations which may reach lengths of from 1 to 4 

 kilometers. According to the detailed accounts of Carez ('03), two groups of intrusions may be 

 distinguished, one belonging to the Lower Lias containing ophite (diabase) only. The second 

 ranges into the Gault, and presents, in addition to the ophite, lherzolite-serpentine, diorite, 

 peridotite, and anorthosite, the latter as a dike in peridotite. Here, then, we find repeated a 

 series of green-rocks which occurs at various horizons through the whole chain of the alpine 

 folds. Steinmann ('05) has drawn attention to the frequent association of these intrusions with 

 the radiolarian shales of the deep sea, but "in the Pyrenees, the south of Spain, and the north- 

 west of Africa they occur in sediments of a kind which is not known in the deep sea, and in this 

 case the facts are not favourable to Steinmann's generalisation. In Europe these green intru- 

 sions or fragmentary traces of them are to be seen in hundreds of localities within regions of 

 great dislocation which extend from the Wolfgang See to the margin of the Sahara. We do not 

 find them in the foreland either in the north or the south, and in Europe we regard them, there- 

 fore, as accompaniments of tectonic movements." ' Lacroix ('01) points out that there is an 

 absolute distinction between the ophites and lherzolites; they are never found passing into one 

 another. The intrusion of the lherzolite occurred subsequent to Lower Cretaceous times, as 

 rocks of that age have been modified by it. It appears in streaked masses with segregation- 

 bands of ariegite, a condition probably resulting from intrusion under great stress. 



Besides the basic rocks we have just described, there are small amounts of an earlier forma- 

 tion. Caralp ('02) has described a series of diabases and picrites among Paleozoic rocks which 

 seem to have some of the characteristics of members of the spilitic suite. 



We will now attempt to follow the line of the alpine folds among which are numerous 



instances of ultrabasic and basic intrusive rocks. Commencing in the Betic Cordillera (Sierra 



de Ronda) , there appears a large lenticular mass of serpentine which, as has recently been 



shown by Orueta and Rubies ('16) and Duparc and Grosset ('16), invades the Cambrian and 



pre-Cambrian schists and gneisses, but is itself of pre-Permian date, for there are abundant 



pebbles of peridotite in the Permian or Triassic conglomerates that unconformably overlie the 



Paleozoic complex. Hence this intrusion, though possibly of Altaid age, was unconnected with 



the alpine folding. 



MOROCCO AND TUNIS. 



The folds of the Betic Cordillera strike across the Straits of Gibraltar and continue along 

 the coast of Morocco and Algeria. 2 This portion of the alpine line of folding presents many 

 remarkable features. There is an inner coastal zone of volcanic rocks, followed by a strip of 

 gneiss and schist, beyond which are the highly folded sedimentary rocks, thrust toward the 

 Sahara. Triassic rocks of the gypsiferous Rotliegende occur with saline beds and limestone, 

 which have been so involved in the orogenic movement that they have been rolled out, myloni- 

 tized, or kneaded together. Large fragments of gneiss and other basement-rocks occur as alien 

 blocks thrust into these softer contorted strata with a great variety of intrusive rocks. Gentil 

 ('02) recognized diabase, diorite, and gabbro, and these are intrusive in every instance, and have 

 produced contact-effects on the Triassic limestone. Almost without exception they have 

 been broken up by tectonic movements and frequently kneaded up with the gypsiferous beds 

 into a breccia. These have been met with throughout the whole range from Morocco to Tunis. 



i The above remarks are a condensation of the account given by Suess (IV, p. 247) . The writer was unable to obtain access to the most important 

 original works on the geology of the Pyrenees, particularly that of Carez, to which reference should be made. 



1 This is the view held by Suess, but it has not been supported by recent work, especially that of Gentil, who has shown that the Riff or African 

 extension of the Betic Cordillera does not continue into the Atlas Mountains. (See La Face de ia Terre VoL HI, 2nd ed. 1921, p. 886, footnote.) 



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