The Geological Lesson. 101 



us. The Potsdam lies directly on the upturned edges of the Lauren- 

 tian rocks, although the seam between the two probably represents 30,- 

 000,000 years as estimated in the table, or gii^a asJikely ten times that 

 figure. During this long period the Trilobite becomes differentiate^!, from 

 some of the worm tribes in some foreign sea, ;anl ^emjgraies'lx) America 

 and Europe during the deposition of the Silurian rocks, leaving behind 

 him his immediate relations and the "connecting links" that would in- 

 dicate his pedigree. 



The same consideration applies to all the life of the Silurian times. It 

 is not likely that any of the tribes originated on the spot. Life had 

 begun on earth probably 100,000,000 of years before that time, and 

 and only spread itself into the west as the conditions for it became pos- 

 sible. When the species become established, there is no difficulty in 

 tracing much of the gradual modification of forms that the varying con- 

 ditions of climate, &c. , worked in them during the long comparatively 

 peaceful ages of the Palaeozoic times. In such cases the blood relation- 

 ship of the modified forms to the old families on the ground is clear 

 enough. Bat where a type differing materially from the old forms 

 makes its appearance we may be sure it is an emigrant. 



This is certainly true of the first fishes that appeared in America and 

 Europe as mentioned above. They arrived in America and multiplied 

 rapidly during the Corniferous period of the Lower Devonian age. But 

 the changes in the next period were unfavorable to them and their colo- 

 nies died out and it was probably half a million years before a perma- 

 nent settlement was effected during the Catskill period. This is like 

 the history of human colonization too. Many of the first efforts to colo- 

 nize America by civilized Europeans were failures. 



All through geological history the tribes and families were going and 

 coming. While they remained here we can trace the changes stamped 

 on the species from period to period. While they were in foreign parts 

 the modifications went on in their structure, habits, &c. , all the same, 

 but we have not in all cases found the detailed record. But when they 

 return to our strata the sum of all the changes they have experienced 

 through many generations of their foreign life is stamped on them and 

 to the first geologists it seemed like a new creation. 



Our first mammals both in America and Europe were little marsupials 

 which appeared in the Triassic and continued to flourish through that 

 and the Jurassic period. At the close of that period the lower rocks 

 were tilted up and subjected during a long period to various oscillations 

 and elevations and in some places denudation, after which they were 

 again sunk beneath the sea and during 9,000,000 years more, from half 

 a mile to 3f miles of Cretaceous rocks were piled on them. During all 

 this time the mammal marsupial has disappeared from this part of the 



