On the Structure and Position of Eozoon Oanadense. 289 



small spaces filled -with carbonate of lime ; and these show the 

 characteristic structure of the fossil. The spaces between them, 

 moreover, are filled with a mixture of serpentine and carbonate 

 of lime. The whole thickness formed by the aggregation of these 

 masses is not less than two hundred feet ; and over their sur- 

 face is spread a sheet of dark-green serpentine, varying from 

 one-sixteenth of an inch to six inches in thickness. This 

 forms the base of a set of newer growths, composed of alter- 

 nating plates of carbonate of lime and serpentine ; the upper 

 surface of which, again, appears to have been worn and broken 

 up by currents and eddies, so as to modify whatever may have 

 been the original surface given by the natural growth of the 

 animal. The difference that presents itself between the 

 deeper and the more superficial parts of the reef, in the fossilizing 

 mineral which has filled up the cavities in the shell that were 

 occupied during life by the sarcodic body of the animal, seems 

 to mark a considerable difference in the conditions under 

 which this substitution took place ; while the fragmentary 

 character of the older pyroxenic portion, and the wear of its 

 surface into cavities and deep recesses, indicate a long period 

 of suspension, during which disintegrating changes were going 

 on, before that renewed growth took place which is represented 

 by the superposed masses wherein the pyroxene is replaced by 

 serpentine. 



A vertical section of a well-preserved mass of Eozoon 

 exhibits in its basal portion a more or less regular alternation 

 of calcareous and siliceous lanielhe; the former being dis- 

 tinguished by their whiteness, the latter by their light-green 

 hue. This alternation, however, frequently gives place in the 

 more superficial parts to a mutual interpenetration of these 

 minerals ; the green spots of the serpentine being scattered 

 over the surface of the section, instead of being collected in 

 continuous bands, so as to give it a granular instead of a striated 

 aspect. This difference depends on a departure from what 

 may be considered the typical plan of growth, which often 

 occurs (as in other Foraminifera) in the later stages ; the 

 minute chambers being no longer arranged in continuous tiers, 

 but being piled together irregularly, or in an acervuline 

 manner. The contrast between the two modes of growth is 

 well shown by the siliceous model of the animal body which 

 occupied the chambers, represented in the Coloured Plate ; the 

 lower portion being that which shows in vertical section a regu- 

 lar series of lam ellge of serpentine, the spaces bet we en which were 

 occupied by lamellae of calcareous shell ; while in the upper is 

 seen the acervuline arrangement of the segments which gives 

 rise to the scattered disposition of the serpentine granules, 

 the calcareous shell having occupied the irregular spaces 

 between these. 



