98 OCCURRENCE AND ORIGIN OF GLAUCONITE. 



indispensable to a right understanding of the minerals, which may 

 be produced when such sandstones are metamorphosed by the action 

 of heat and pressure, so as to become reconverted into crystalline 

 rocks, such as schists ; for the presence of felspar, iron, and many 

 other minerals, renders the composition and variety of such schists 

 readily intelligible. 



Glauconite. 



This is one of the most characteristic minerals found in sand. The 

 grains arc usually dark green, amorphous, and on being powdered 

 yield a bright-green colour; frequently they invest the cells of 

 foraminifera. 



Glauconite is a double silicate of iron and alumina, with a certain 

 amount of the alumina replaced by magnesia, soda, and potash. Its 

 composition is nearly identical with seladonite, the green earth found 

 in the vesicular cavities of certain basaltic rocks. It is known to be 

 produced by the alteration of augite and hornblende, but in some of 

 the rocks it appears to have formed by replacing particles of yellow 

 ferruginous mud. Its origin is concretionary, and it is probably 

 formed at the time of deposition. At the present day considerable 

 deposits of glauconite are found off the coast of North America, so 

 that their existence was attributed by Professor Kogers to the influence 

 of an ancient Gulf Stream. Similar deposits have been met with 

 during the " Challenger " exploration off Portugal, off the east coast of 

 Australia, and off the Crozet Islands, at depths of 400 to 600 fathoms ; 

 and it may certainly be inferred that a stream flowing in the ocean from 

 a warmer to a colder region would inevitably have a tendency to pre- 

 cipitate certain of the mineral substances which the water originally 

 held in suspension when its temperature was higher. Glauconite occurs 

 in the green slates of the English Lake district, where it fills the cells 

 in fragments of pumice ; a similar material is found in the slate of 

 Penrhyn. But glauconite is most characteristic in the various Secon- 

 dary and Tertiary sands. In the Lias, sandy beds of the Forest Marble, 

 Calcareous Grit, Portland Rock, ISTeocomian Sand, and Upper Greensand, 

 it often gives a colour to the deposit. It is found at the base of the 

 Thanet Sands, and throughout those beds, and in the Bracklesham 

 Beds, and Barton Clay. It sometimes mineralises fossils. Its abundance 

 and constant repetition in the rocks of the South of England strongly 

 suggest that its existence is due to the same physical causes, such as 

 the denudation of like plutonic rocks. Nevertheless, in chemical 

 composition it is curiously similar to the residue of chalk which would 

 be left after the carbonate of lime has been removed ; and since there 

 is abundant evidence that the carbonate of lime of the Chalk has been 

 removed subsequent to the deposition of the Tertiary beds so as to 

 interpose a stratum of unworn flints on top of the Chalk, Professor 

 M'Kenny Hughes has argued that the glauconite in which those flints 

 are embedded has been formed in situ out of the residue of the Chalk, 

 and may be now forming. 



