438 E. S. MOORE 
time, since it has been generally agreed among geologists who have 
seen the same series of rocks on the east coast of the bay that they 
are pre-Cambrian in age. 
The iron-formation consists of jasper, chert, hematite, mag- 
netite, siderite, and green granules regarded as the iron silicate, 
greenalite. The chert and hematite are also in concretionary form, 
and it is suggested that the algae and iron bacteria have been 
responsible for the precipitation of colloidal silica, hematite, and 
iron silicate in this granular form, in some places as a direct pre- 
cipitate on the floor of the basin and in others as a replacement of 
the calcite granules by the iron compounds. 
