398 E. C. HARDER AND R. T. CHAMBERLIN 



quartzite is thick. Following the Caraga quartzite is the Batatal 

 schist, the finer material of which implies a slackening in the vigor 

 of sedimentation, whether because the land from which the sedi- 

 ments were being derived was becoming lower, or because the 

 source of the sediment was becoming more remote owing to an 

 advancing shore line, or because of deepening of waters or shifting 

 of currents. The Batatal schist is thin, indicating that this stage 

 was not long enduring. It was followed by local flows of basic lava. 

 And then came the deposition of the iron formation. The hypothe- 

 ses to explain this remarkable formation naturally are of two sorts : 

 (i) that the iron oxide was a mechanical sediment washed in like 

 the mud of the schist and the sand of the quartzite; (2) that it 

 was precipitated from solution through either chemical or bio- 

 chemical agencies. 



Following the hypothesis that the iron oxide was a mechanical 

 sediment, the source for the iron oxide should naturally be the 

 residual material resulting from the thorough weathering of the 

 rocks of the basement complex. Besides this hydrated iron oxide, 

 the residual material should contain kaolin and quartz in some- 

 what greater proportion than the iron oxide. Under any hypothesis 

 the sediments which resulted in the Caraga quartzite and the 

 Batatal schist came from the sorting of thoroughly weathered 

 material by stream and wave action. The Caraga quartzite repre- 

 sents a time when only material of the coarser sort, the sand 

 grains, could find rest in the area under consideration. The finer 

 material was swept farther out to sea to find lodgment in deeper and 

 quieter waters. With the lessening of the vigor of the agents of 

 sedimentation came the thin Batatal schist. At this time fine mud 

 was being washed into the sea and deposited, while the coarser sand 

 was either not washed into the sea or was left still nearer the shore 

 which may have changed its position in the meantime. With the 

 kaolin much iron oxide must also have been washed into the sea and 

 not a little of it deposited with the mud. 



Thus far the process has been the ordinary one of decay of 

 igneous rocks and the deposition of much of the resulting material 

 as clastic sediment. The unusual feature to be explained is what 

 very peculiar condition obtained to cause the deposition of ferric 



