MIDDLE CAMBRIAN. ]5 



SILICIFICATION. 



To the biologist the suggestion of silicified medusae is a violent attack 

 upon his previous conceptions of such organisms and the possibilities of their 

 preservation as fossils in any other manner than as faint impressions on fine 

 limestone, sandstone, or shale. The fact is, however, that they occur in a 

 silicified condition, and we now have to consider how this may have come 

 about. 



If the views expressed in the foregoing section on the "Condition and 

 manner of preservation" (pp. 4-7) are correct, the Middle Cambrian medusae 

 under consideration lived in relatively shallow water, not far from the shore 

 line. It is also inferred that they were quickly overwhelmed and buried in 

 a siliceous mud that was subsequently consolidated to form a siliceous shale. 

 There are certain features, however, connected with the external appearance 

 of the nodules which strongly indicate that they were exposed for some 

 time near to or directly on the bed of the sea. The former is more prob- 

 able, as it is difficult to give any explanation of the preservation and silicifi- 

 cation of an organism like a medusa on the bed of the sea. Nature performs 

 so much of her chemical work in the consolidating mud or ooze just below 

 the surface, that this has been aptly called the "chemical laboratory of the 

 sea." The external features referred to are the presence of casts of annelid 

 trails, entire trilobites, brachiopods and pteropods (PI. XV, figs. 1 and 5; 

 PI. XVI, figs. 6, 7, 8) attached to the outer surface of the nodule, and medusa- 

 like those represented by figs. 7 and 8 of PL XVII, figs. 3 and 5 of PL XXI, 

 and figs. 2, 3, 5, and 6 of PL XXII. In many instances the medusa is 

 entirely buried in the nodule; in others partially, and often only a small 

 amount of matter was deposited between the lobes and about the oral arms. 



The mode of occurrence of the tests of trilobites, usually in fragments 

 but often entire, suggests the formation of a nodule about a fragment, or a 

 mass of fragments, resting on or embedded in the ooze. That the nodules 

 did not reach a larger size than 20 cm in diameter is probably owing to the 

 rapid deposition of mud on and over them, 1 or to the consolidation and 

 hardening of the sediment in which they were forming before the silicifying 

 solution could build up a larger nodule. 



1 1 have observed masses of fossils and calcareous matter in the Niagara calcareous shales of 

 Waldron, Indiana, some of which were several feet (2 meters or more) in diameter and a foot or two 

 (one-half meter or more) in thickness. 



