FORAMINIFERA. 



117 



The most important deposit of this nature at the present day is the 

 well-known " Globigerina ooze," which is found to cover vast areas 

 of the sea-bottom in all the great oceans, mostly at depths of from 

 600 to about 2500 fathoms, and which is of special interest from the 

 resemblances which it presents to the White Chalk. The " Globi- 

 gerina ooze " is a whitish or greyish mud, principally composed of 

 carbonate of lime, but containing a considerable proportion of silice- 



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MMmm 



l A':7r0 L 



Fig. 26. — a, Diagram of one of the higher forms of the vitreous Foraminifcra, showing the 

 double nature of the septa (b), the stolon-passages between successive chambers {a), and the 

 supplemental skeleton (d) ; b, Test of Calcarina Sficngleri, magnified twelve diameters, show- 

 ing the spines formed by the supplemental skeleton ; c, Part of a section of the test of 

 Calcariua, magnified fifty diameters, showing the tubulated " proper walls " of the chambers 

 (u), and the canal-system of the intermediate skeleton (d) ; d, Part of the test of Niemmulina 

 IcEZ'igata, highly magnified, showing the canal-system of the septa (s), and marginal cord («). 

 (After Carpenter.) 



ous matter, along with a variable quantity of subordinate ingredients 

 such as alumina. When examined microscopically, the Globigerina 

 mud (fig. 27) is found to consist principally of the entire or broken 

 shells of Foraminifera, belonging mostly to the genera Globigerina, 

 Orbulina, Hastigerina, and Pidvinitlina, together with many of the 

 singular calcareous organisms known as " coccoliths." In the nature 

 of the organisms of which it is composed, the Globigerina ooze of the 

 present day presents the closest resemblance to White Chalk ; while 

 the chemical differences between them — especially as regards the 

 relative percentage of silica — admit, as will be more particularly 



