288 
ELEMENTS OF GEOLOGY. 
Fig. 2^.6. 
Fig. 232. 
Fig. 233. 
Fig. 234. Fig. 235. 
Fig. 232. GloMgerina bulloides. Calcareous Rhizopod, 
“ 233. Actinocyclus. ) 
“ 2M. Pinnularia. y Siliceous Diatomacece. 
“ 235. Eunotia bide7is.) 
“ 236. Spicula of sponge. Siliceous sponge. 
viscid, chalky mud, wholly devoid of Globigerinae. This mud 
was perfectly homogeneous in composition, and contained no 
organic remains visible to the naked eye. Mr. Etheridge, 
however, has ascertained by microscopical examination that 
it is made,up of Goccoliths,^ Discoliths,^ and other minute fos¬ 
sils like those of the Chalk classed by Huxley as Bathybius^ 
when this term is used in its widest sense. This mud, more 
than three miles deep, was dredged up in lat. 20° 19' N., 
long. 4° 36' E., or about midway between Madeira and the 
Cape of Good Hope. 
The recent deep-sea dredgings in the Atlantic conducted 
by Dr. Wyville Thomson, Dr. Carpenter, Mr. Gwyn Jeffreys, 
and others, have shown that on the same white mud there 
sometimes flourish Mollusca, Crustacea, and Echinoderms, be¬ 
sides abundance of siliceous sponges, forming, on the whole, 
a marine fauna bearing a striking resemblance in its general 
character to that of the ancient chalk. 
Popular Error as to the Geological Continuity of the Creta¬ 
ceous Period.-— We must be careful, however, not to overrate 
the points of resemblance which the deep-sea investigations 
have placed in a strong light. They have been supposed by 
some naturalists to warrant a conclusion expressed in these 
words: ‘^We are still living in the Cretaceous epoch;” a 
doctrine which has led to much popular delusion as to the 
bearing of the new facts on geological reasoning and classi¬ 
fication. The reader should be reminded that in geology 
we have been in the habit of founding our great chronolog¬ 
ical divisions, not on foraminifera and sponges, nor even on 
echinoderms and corals, but on the remains of the most 
highly organized beings available to us, such as the mollus¬ 
ca; these being met with, as above explained (p. 142), in 
stratified rocks of almost every age. In dealing with the 
mollusca, it is those of the highest or most specialized organ-* 
ization which afford us the best characters in proportion as 
their vertical range is the most limited. Thus the Cephalo- 
