319 



seems to prove a remarkable uniformity of conditions as 

 prevailing either at the time of deposition or subsequently. 



2. The origin of the quasi-oolitic bodies, as distinct from 

 the crystalline groundmass, is not easy of determination. The 

 differences between these and typically-formed oolitic grains 

 have already been noted (page 314). Tlie presence of 

 hydrous oxide of iron, generally distributed throughout the 

 bed, as well as in patches and as a cementing agent, is sugges- 

 tive of their origin. Assuming this much, it remains to be 

 proved as to whether they were formed originally, at the time 

 of deposition (syngenetically) , or at some later period when 

 the rock was undergoing molecular reconstruction by 

 metamorphic action (enterolithic). The evidence, so far as 

 available, appears to favour the latter view. The segregation 

 of iron oxide in sand and other sediments, forming ''iron 

 balls" and other concretions by cementation, is a well-known 

 circumstance. Th^se are common m the superficial beds of 

 Central Australia. A similar segregation of iron oxide under 

 the reconstruction of a rock by crystallization has been 

 observed by the writer in the silicification and calcification of 

 the plants of the coal measures of the north of England. In 

 some cases of silicification the ferruginous material formed 

 circles (or spheres) without obliterating the organic structure; 

 while in the process of conversion to a calcic radial crystalliza- 

 tion the cellular structure became destroyed (in places) and 

 the ferro-carbonaceous material formed dark borders between 

 contiguous radial crystalline areas, and also a dark nucleus at 

 the centre of each of these crystalline areas. In the proto- 

 metamorphic process the limestone of the Brighton horizon 

 became crystallized, and we may assume that the matter 

 foreign to the calcium carbonate was excluded from the calcite 

 crystals, forming centres of cementation among the original 

 particles of the limestone and arrested the process of 

 crystallization within such cemented areas. The fact that 

 some of the spherules in the limestone possess a nucleus of 

 clear crystalline calcite is suggestive that the sub-oolitic 

 structure was coincident with and arose out of the condition 

 created by the metamorphic action. 



3. The brecciation is an interesting feature, especially as 

 developed in the Depot Creek outcrops. The angularity of 

 many of the included fragments suggests a mechanical force 

 which must have operated subsequently to the formation of 

 the sub-oolitic structure, as no distinction exists as to form 

 and mode of occurrence between those included in the 

 fragments and those in the crystalline matrix. More than 

 one period of brecciation appears to have occurred, as smaller 

 angular fragments are included in some of the larger ones. 



K 



