THE PEESEEVATION OF EOZOON. 99 



are empty^ or only partially filled^ wliicli is sometimes 

 the case even with Eocene Nummnlites and Cretaceous 

 forms of smaller size^ — are very frequently filled solid 

 with calcareous matter, and as Dr. Carpenter well 

 remarks_, even well preserved Tertiary Nummulites 

 in this state often fail greatly in showing their struc- 

 tures, though in the same condition they occasionally 

 show these in .great perfection. Among the finest 

 I have seen are specimens from the Mount of Olives 

 (fig. 19), and Dr. Carpenter mentions as equally good 

 those of the London clay of Bracklesham. But in 

 no condition do modern Foraminifera or those of the 

 Tertiary and Mesozoic rocks appear in greater perfec- 

 tion than when filled with the hydrous silicate of iron 

 and potash called glauconite, and which gives by 

 the abundance of its little bottle-green concretions 

 the name of ^' green-sand^' to formations of this age 

 both in Europe and America. In some beds of green- 

 sand every grain seems to have been moulded into 

 the interior of a microscopic shell, and has retained 

 its form after the frail envelope has been removed. 

 In some cases the glauconite has not only filled the 

 chambers but has penetrated the fine tubulation, and 

 when the shell is removed, either naturally or by the 

 action of an acid, these project in minute needles or 

 bundles of threads from the surface of the cast. It 

 is in the warmer seas, and especially in the bed of 

 the ^gean and of the Gulf Stream, that such specimens 

 are now most usually found. If we ask why this 

 mineral glauconite should be associated with Foramini- 



