DISTRIBUTION OF FOBAMINIPERA. 237 



the most striking of the faunal features of the period, and, aside 

 from the general importance attaching to the genus itself as a 

 rock-builder, acquires special significance from the circumstance of 

 the barrenness in foraminiferal remains of the formations immedi- 

 ately preceding. From the close of the Paleozoic period to the 

 present day there is a steadily increasing development of distinctive 

 types, and an equally steady approximation to the structural type 

 which dominates the modern seas. Almost every period is marked 

 by some extensive foramiuiferal accumulation, and in nearly all 

 cases the distinguishing character is given to these accumulations 

 by genera which acquire successive importance. Such are the 

 Gyroporella limestones of the Triassic and Rhsetic formations, the 

 miliolite and nummulitic rocks of the Eocene and Oligocene, and 

 the orbitoide rocks of the Oligocene and Miocene periods. In the 

 newer Tertiary deposits the representative genera are mainly identi- 

 cal with those inhabiting the modern seas, and even of the species 

 a fair proportion are identical. Of the most widely distributed 

 and most numerously represented modern genera the one which 

 has most thoroughly left its impress upon the rocks of past . ages is 

 the genus Globigerina. Beginning in the Trias, it already attains 

 to considerable importance in the Rhaetic, where, in association 

 with Textularia, Orbulina, and Quinqueloculina, it forms massive 

 limestones (Dachsteinkalk, near Hallstadt, in Salzburg). The chalk 

 is made up in large part of the three foraminiferal genera Glo- 

 bigerina, Textularia, and Rotalia, and the first of these, as has 

 already been seen, is the principal constituent also of the modern 

 Globisrerina or Atlantic ooze. 



o 



Much diversity of opinion has existed among naturalists as to 

 the mutual relationship of these two classes of deposits the chalk 

 and ooze. The general similarity of lithological character, height- 

 ened by a close identity existing between the contained faunas, has 

 led most scientists to believe that the present oceanic bottom is 

 but a continuation of the bottom of the Cretaceous seas in other 

 words, that the Cretaceous epoch is continued up to the present 

 time without any very material change marking the progress of 

 organic life in the ocean's deepest parts. The majority of the 

 foraminiferal types occurring in the one formation are represented 

 in the other, and the identity is carried even to a fair proportion of 

 the species, Further, Sir Wyville Thomson has shown that the 



