!9 18 ] Davis: The Radiolarian Cherts of the Franciscan Group 385 



HYPOTHESIS THAT THE SILICA OF THE CHEETS IS OF MAGMATIC 

 OBIGIN PEECIPITATED BY EADIOLAEIA 



Another idea regarding the origin of the radiolarian cherts was 

 proposed by Dewey and Plett 149 in connection with a discussion of 

 pillow lavas. They show that the pillow lavas belong to the group of 

 spilites. This is a group of basic igneous rocks of varying types, but 

 always characterized by soda-rich feldspars. In the British Isles, 

 spilitic lavas have been erupted repeatedly and on a large scale. 

 Associated with them, are rocks which are believed to represent the 

 intrusive members of the spilitic group. 



The spilitic rocks are as a rule very completely decomposed 

 throughout their whole mass. The thoroughly decomposed character 

 of the rocks and of most of the feldspars which they contain, makes 

 the occurrence of fresh albite in them a peculiar feature. It is be- 

 lieved that the formation of albite and the decomposition of the rock 

 is an after-phase of vulcanism. 



Tending to show this supposition correct, is a peculiar feature of 

 the contact zones, produced in these rocks where they have been 

 intruded by granite. The lavas are of Lower Carboniferous age and 

 the granite was intruded during the Upper Carboniferous. Yet at 

 the time of the intrusion of the granite the spilitic rocks were greatly 

 decomposed, and the vesicles were filled by secondary minerals. This 

 is shown by the character of the minerals developed in these vesicles 

 during the contact metamorphism. Also, the new basic feldspars, 

 developed in these rocks as a result of the metamorphism by the 

 granite, have not been greatly altered since Carboniferous time. It 

 would then appear that the thorough-going alteration in Carbonifer- 

 ous time was produced by some agent more powerful than ordinary 

 weathering. 



The albite diabases, which occur in association with the spilitic 

 lavas, and which represent an intrusive member of the spilitic group, 

 have produced a considerable amount of alteration of shale to adi- 

 nole. Adinoles when most perfectly developed consist of nearly pure 

 albite. No shales are so rich in alkalies as these adinoles, so that 

 there is no escape from the conclusion that soda has been added to the 

 shale at the contact of the albite diabases. 



The formation of adinoles by the albite diabases and the albitiza- 

 tion of the extrusive spilites make it appear certain that during the 



i*» On some British Pillow Lavas and the Eocks associated with them, Geol. 

 Mag., vol. 48, p. 202, 1911. 



