MIDDLE CAMBRIAN. 17 
The analysis of the shale shows only a trace of caleareous 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 medusz 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 
might 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 meduse, or any other 
organic forms, it appears that nodules of siliceous matter were also formed 
independently of the presence of the medusz. 
In the case of the medusz 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. ither 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 medusze 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 
