i8 9 4-95-] T 97 



300 fathoms, but it is found occasionally as far down as 2000 

 fathoms. It is usually accompanied by such minerals as quartz, 

 orthoclase, mica, garnet, epidote, etc., and fragments of ancient 

 rocks, such as chloritic rocks, granite and mica schists, etc., and 

 in modern deposits by organic matter of vegetable origin and 

 with phosphate of lime. It never occurs in true volcanic muds 

 and sands, and it is hardly ever found in pure carbonate of lime 

 deposits, but is always associated with sandy calcareous deposits, 

 and is most probably formed in situ and not conveyed like the 

 land rocks always found with it, and from which it is most 

 likely formed by the prolonged action of the sea water. The 

 conclusion arrived at in the " Challenger " Report is that 

 Glauconite is probably initially formed in the cavities of cal- 

 careous organisms by infiltration, the composition of the mineral, 

 which is really a mixture, being determined in part by the 

 organic matter in the shell. 



We come now to our own local specimen, the following 

 details have been given me by Miss Thompson : — 



"The Glauconite grains occur in a cliff of the so-called 

 chloritic sandstone division of the upper greensand, which 

 belongs to the Cretaceous Period. This cliff is part of the valley 

 which has been cut in the Knockagh Mountain by the Wood- 

 burn River ; or more simply, the Woodburn River has cut 

 through the Cretaceous rocks and exposed this face of highly 

 fossiliferous greensand." 



The method adopted for obtaining the grains was this: — The 

 rock was pulverized, washed in a muslin bag until the water 

 ran off tolerably clear, the powder dried and sifted through 

 coarse muslin, and then with the help of a lens of low magni- 

 fying power the grains of Glauconite were picked up by means 

 of a very fine sable brush moistened in a tiny vessel of distilled 

 water, into which the collected grains off the brush were 

 dropped. The field of the lens was large enough to admit of 

 this little vessel of water and a tray of glossy brown paper (upon 

 which pinches of the powdered rock were shaken) at the same 

 time, so that no time was lost in dropping the Glauconite into 



