78 PROCEEDINGS OF THE GEOLOaiCAL SOCIETY. 



equilibrium, like those in fresh volcanic ashes, it is equally clear 

 that great subsequent changes might be expected. Thus, for ex- 

 ample, in a section of consolidated peperino from Vesuvius, I find 

 that the fragments of augite have been altered into a new substance, 

 and the cavities in the ash more or less completely filled with a 

 fibrous mineral with feeble double refraction and having the 

 negative axis of depolarization in the line of the fibres. My belief 

 is that this is some hydrous silicate — perhaps a zeolite ; but at all 

 events its formation has thoroughly hardened the whole deposit, 

 and given rise to a greater change in the structure than is met 

 with in many rocks of remote geological epochs. 



An almost colourless crystalline mineral has also been developed 

 in some of the mud from 2230 fathoms in the South Pacific. It is 

 insoluble in strong hot acids until after it has been boiled in 

 caustic potash, and is probably a silicate. It occurs in small 

 crystals, which have been deposited in radiate groups on the surface 

 of many of the sand-grains. The mud also contains many frag- 

 ments of an apparently basic lava, more or less completely altered 

 into a sort of palagonite ; and probably the crystalline mineral was 

 a product of this change. 



Some specimens of our gault, when washed, yield analogous 

 radiating groups of a transparent mineral insoluble in acids. 



Another silicate very commonly found in stratified rocks is 

 glauconite. According to Ehrenberg it occurs inside the cells of 

 Poraminifera in some recent deposits ; it is at all events well seen 

 in many of our older rocks. I have studied it more particularly 

 in the harder bands of the Barton Clay, in the firestone of Yentnor, 

 in the Kentish Eag, in the Greensands, in Lias, and in some other 

 rocks. It is often found filling the cells of Eoraminifera, the 

 central hollow and minute tubes in the joints of Pentacrinus, 

 the open spaces of corals, holes bored into shells, open spaces 

 left amongst crystals of calcite when aragonite shells became 

 crystalline, and what were probably minute borings present in 

 the deposit at the time of its formation ; in fact, it may be said 

 to fill up cavities of all kinds. In some few cases it appears as if 

 it had crystallized out independent of any cavity ; but often there 

 is no positive evidence of how it was formed. 



As far as I have been able to ascertain, glauconite consists of 

 more or less irregular and imperfect scales, having the negative 

 axis perpendicular to their surface. ' Usually these scales are ex- 

 tremely minute, and are arranged in no definite order ; but occa- 

 sionally they are larger, sometimes arranged in a radiate, and some- 

 times in ■ a concentric manner. It is very curious to see in the 

 sections of some rocks how the grains of glauconite have been 

 crushed by pressure, though the fragments still remain scattered 

 about in close proximity. 



Since glauconite is known to occur as a product of the alteration 

 of augite and hornblende, and since in the cavities of some altered 

 dolerites I have found a green substance having exactly the same 

 characters as the glauconite of the Greensand, it would be a very 



