EEPORT ON THE DEEP-SEA DEPOSITS. 
385 
North America. This abundance of glauconite is continued into the Tertiary formations, 
from the lowest up to the highest horizons of the series. 
From this rapid enumeration it will be seen that glauconite traverses the whole of 
the geological periods, and its formation is continued in modern deposits along many 
continental shores explored by the Challenger and other expeditioDS. A remarkable 
analogy is also found between the size of the grains now formed in marine deposits and 
that of the grains found in the geological series of rocks. It has been stated that some 
of the grains in the primary formations are of very large size — several centimetres in 
diameter. All the specimens of this kind, however, which we have been able to examine 
are found, on microscopic examination, to be made up of an agglomeration of grains rarely 
exceeding a few millimetres in diameter, and therefore closely resembling the glauconitic 
nodules or aggregations dredged by the Challenger on the Agulhas Bank off the Cape 
of Good Hope, in depths of 100 and 150 fathoms. 
It is also important here to point out the association that exists in geological formations 
between glauconitic and sandy calcareous deposits, and the absence or rarity of glauconite 
in formations of pure chalk, or in nearly pure carbonate of lime deposits ; glauconite may 
therefore be regarded as having been formed either in deep water not far from the coasts 
or in shallow water at parts of the coast where no large quantity of continental debris was 
deposited. This fact is significant, as it appears to prove the coast and subcoast character 
of these glauconitic deposits in past geological times, which consequently present a com- 
plete analogy with the glauconitic deposits of modern seas, both with respect to the 
conditions under which they were formed and their mineralogical composition. These 
analogies likewise prove the continuity of geological phenomena and the presence of 
nearly identical conditions in the sea during long periods in the history of the globe ; they 
indicate that the presence of terrigenous matters, directly derived from the disintegration 
of continental land, is a necessary condition for the formation of glauconite, and this fact 
must be taken account of in any discussion bearing upon the origin of this mineral. 
Chemical Composition and Mode of Formation . — While it must be admitted 
that we have arrived at certain definite and satisfactory conclusions as to the condi- 
tions under which glauconite is found in our present seas, as well as in geological 
formations, we are far from having at our disposal all the facts necessary for a complete 
explanation of its mode of origin. So many possible reactions may take place in the 
deposits being laid down in existing seas, that it is difficult to be certain that any one of 
them is necessarily the one which has been followed in the deposition of this silicate in 
the terrigenous deposits. The explanations that are given with reference to the formation 
of glauconite must then be more or less hypothetical ; it is not to be wondered at that 
its origin has remained for a long time enigmatical, and that the researches of numerous 
mineralogists up to the present time have not led to any very definite results. The 
chemical analyses of glauconite have been very numerous ; but, from the nature of the 
(deep-sea deposits chall. exp. — 1891.) 49 
