MIDDLE CAMBRIAN. 17 



The analysis of the shale shows only a trace of calcareous matter and 

 53 per cent of silica. The nodules are almost wholly siliceous, the calcite 

 occurring as microscopic points embedded in the siliceous matter. An 

 analysis of a portion of a nodule, within the body of a medusa, shows 

 88.33 per cent of silica and 3.91 per cent of calcite. If the sediment which 

 now forms the shale buried the medusae in it, the process of mineralization 

 must have gone on within the highly siliceous mud. If the sediment set 

 quickly it is possible that the form of the medusa was preserved, and that 

 the silica was deposited directly from the alkaline silicates precipitated in 

 connection with the presence of organic matter. This would account for 

 the formation of the siliceous nodules without the intervention of calcite 

 and the secondary replacement by silica. The process of silicification 

 mio-ht then have taken place in the manner suggested, and, when this had 

 once begun, additional deposits might have been made so as to form a 

 nodule or concretion about the medusa. From the presence of a large 

 number of nodules in which there are few traces of medusae, or any other 

 organic forms, it appears that nodules of siliceous matter were also formed 

 independently of the presence of the medusae. 



In the case of the medusae preserving the interior canals, it is probable 

 that the canals and internal cavities of the medusa were filled at once, to a 

 greater or less extent, by the soft siliceous ooze or mud. As the animal 

 matter decomposed, the ooze gradually took its place, and then began the 

 silicification of the sediment that resulted in the formation of the cast of 

 the medusa, and of the nodules by the extension of the silicification into 

 the surrounding sediment. Either before or while this was going on, the 

 annelids bored their way through the medusa and the surrounding matrix. 

 This appears to be the most probable explanation of the preservation of 

 the medusae and the formation of the nodules. 



SOURCE OF THE SILICA. 



The silica that forms so large a portion of the shale is probably derived 

 from the original detrital quartz, which occurs in microscopic grains, and 

 from that deposited in the sediment from the solution of siliceous organ- 

 isms that were buried in the mud or were present on the bed of the sea. 

 The soluble silica undoubtedly furnished some of the silica that formed 

 many of the nodules. It may be urged that the silica was deposited directly 

 mon xxx 2 



