Stevenson.] '±'*^ [June IS, 



along the sea-border, exposed to frequent irruptions of sea-water. The 

 lacustrine, or purely fresh-water deposits, are small both in extent and 

 duration, and are confined chiefly to the later portions of geological time. 

 As the sea always covered the greater part of the earth and afforded an 

 easy medium of migration for water-breathing animals, one would expect 

 to find in the rocks of marine origin the most satisfactory record of 

 changes in animal life. This would be a close record of changes in physical 

 conditions, for animals are of a high type of organization, and, therefore, 

 very sensitive to alteration of circumstances. The record is remarkably 

 complete. From the base of the Silurian to the present time the gaps are 

 few and usually of limited extent. In our country there is no group of 

 rocks, excepting one, which does not yield a plentiful supply of inverte- 

 brate remains over perhaps the greater part of its area. Even the Trias- 

 sie, usually so barren in America, is at many localities rich. 



So distinct is the succession of invertebrate life, so sharp .the breaks at 

 the close of many periods in the world's history, that geologists by com- 

 mon consent have adopted this form of life as the foundation-stone of our 

 system. By sti-atigraphy the succession of the rocks was determined, 

 but by the succession of invertebrate life the great mass was divided 

 into groups and geological history could be written. Rocks containing 

 a certain fauna were called Silurian, others with a different grouping 

 were termed Cretaceous, and others Miocene. These divisions were made 

 on the basis of the fauna and on no other basis. This should be borne in 

 mind. 



The same succession is employed in making the minor divisions. In 

 the Upper Missouri Region a mass of rocks is found, possessing a fauna 

 closely resembling that of a series in Europe, termed Upper Cretaceous. 

 This, all accept as proving that the two series occupy equivalent positions 

 in the geological succession. Closer investigation shows that the Upper 

 Missouri series is made up of five distinct groups, each characterized over 

 an immense area by a peculiar assemblage of invertebrate remains. These 

 groups make the section. If in any portion of the whole Western region 

 we find the fossils of any one of these groups in a mass of rocks, we may 

 legitimately expect to find the others over or under it, as the case may 

 be. It may occur that over large areas a group thus established is per- 

 fectly barren of animal remains. This does occur in the Cretaceous 

 groups. The Dakota Group is often barren, and can be identified only 

 by its previously determined stratigraphical relations. The Fort Pierre 

 and Fox Hills Groups, we are told by Dr. Hayden, show extensive zones 

 of barrenness, whereas they are generally prolific. To explain this varia- 

 tion is not always easy, but we cannot do it by any assumption that the 

 prolific portions mark the site of lagoons held by elevation and contain- 

 ing a few relics of a past age. In some instances these "lagoons " would 

 involve us in difficulty, as the fossiliferous layers in different zones occupy 

 different horizons, so that the past age, whose fauna was preserved in 

 the "lagoons," would need to be "jDast" and "present " alternately for 



