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University of California Publications in Geology [Vol. 11 



greatly reduced by the precipitation of magnesium silicate and that 

 after this had occurred pure silicic acid would come down. 



This idea of the origin of the cherts accounts for the world-wide 

 association of radiolarian cherts with this particular type of igneous 

 rock, and also for the frequent association with glaucophane schists. 

 The constant association of these rocks implies some genetic relation 

 between them. 



There are exceptions to the above statement, but they are very 

 rare. For example, the novaculites of Arkansas, Texas and Oklahoma 

 do not appear to be associated with igneous rocks of this sort. Also 

 certain occurrences of ellipsoidal basalts show no association with 

 cherts. 



Another mode of accounting for the frequent association of the 

 two types of rocks is a modification of Harker 's idea that tectonic con- 

 ditions affect the type of igneous activity. It is conceivable that the 

 radiolarian cherts may represent radiolarian oozes and that in the 

 abyssal depths where such oozes accumulate the tectonic conditions 

 might be favorable for the eruption of igneous rocks of this peculiar 

 type. Steinmann 171 has applied this idea to certain occurrences in 

 the Alps. He believes that under the great depths of the ocean, there 

 are bodies of gabbro-peridotite magmas, and thinks that when the 

 abyssal sediments are raised above the surface of the sea, these basic 

 magmas are injected into the sediments. However, the idea is not 

 very convincing, since it encounters the fact that the cherts are not 

 abyssal sediments. 



Another fact which is in harmony with this idea of the origin of 

 the radiolarian cherts is that in none of the oceanic investigations has 

 anything at all like the radiolarian cherts been discovered. It has 

 been shown that they are not like radiolarian oozes nor like any 

 type of rock which might be derived from these oozes. In spite of 

 the absence of these present-day equivalents, radiolarian cherts while 

 certainly not abundant, are yet found in many places in the world. 

 If formed under conditions of ordinary sedimentation, either mechan- 

 ical, organic, or chemical, one would expect somewhere to encounter 

 their equivalents in process of formation. The fact that such has not 

 been found points to the conclusion that the formation of these cherts 



171 Steinmann, G., Geologische Beobachtungen in den Alpen. II, Die Schardt 'sehe 

 Ueberfaltungstheorie und die geologische Bedeutung der Tiefseeabsatze und der 

 ophiolitischen Massengesteine. 



Abstract by O. Wilekens in Neues Jahrbueh f. Mineralogie, Geologie und 

 Palaontologie, Jahrgang II, 92, 1906. 



