I96 [Proc. B.N.F.C. 



generally containing varying proportions of other bodies, such 

 as alumina, lime, and magnesia. It occurs extensively dis- 

 seminated in small opaque grains, especially in the lower parts 

 of the chalk marl, chloride marl, and greensand; but it is found 

 also at different horizons in the whole geological series of rocks 

 from the Cambrian up to the most recent Tertiary layers, and, 

 indeed, is particularly interesting in being one of the very few 

 silicates which are in actual process of formation on the sea bed 

 at the present time. The physical characteristics of the grains 

 are practically the same throughout the series ; the normal 

 colour is dark green, but sometimes appears as yellowish, greyish, 

 or even almost red. These variations of colour, however, mean, 

 at least, the commencement of decomposition. The size of the 

 grains is usually about one millimetre in diameter, and although 

 much larger masses are sometimes found these are merely 

 agglomerations of the smaller grains. The hardness is about 

 that of rock salt. Under the microscope the grains appear 

 quite homogeneous unless some foreign body is enclosed, or, as 

 sometimes occurs, the commencement of decomposition gives a 

 more or less zonary appearance to them. 



The mode of formation is a much contested point, and is not 

 yet decided ; but, as bearing upon this point, the following 

 remarks culled chiefly from the " Report on Deep Sea Deposits" 

 of the " Challenger " expedition may be of interest : — 



Glauconite is found not universally distributed over the 

 floor of the ocean but in deposits along continental shores and 

 not in the deep pelagic ones. In marine deposits it is always 

 associated with the casts of Foraminifera, and roughly resembles 

 these in form. It is characteristic of, and sometimes abundant 

 in, green muds and sands, and also in Globigerina ooze, especially 

 when this contains a good deal of detritus from neighbouring 

 continental land. It is not found in true pelagic deposits, nor 

 is it usual where the detrital matters from rivers are abundant, 

 but along the high bold coasts where accumulation is slow 

 Glauconite is found in typical form and great abundance. 



The usual depth at which Glauconite occurs is from 200 to 



