832 THE POPULAR SCIENCE MONTHLY. 



of Laurentian rocks, and others of quartz-pebbles, which must have 

 been the remains of rocks subjected to very perfect erosion. The 

 pure quartz-rocks tell the same tale, while limestones and slates speak 

 also of chemical separation of the materials of older rocks. The Hu- 

 ronian evidently tells of movements in the previous Laurentian, and 

 changes in its texture so great that the former may be regarded as a 

 comparatively modern rock, though vastly older than any part of the 

 paheozoic series. 



Still later than the Huronian is the great micaceous series called 

 by Hunt the Mont Alban or White Mountain group, and the Taco- 

 nian or lower Taconic of Emmons, which recalls in some measure the 

 conditions of the Huronian. The precise relations of these to the later 

 formations, and to certain doubtful deposits around Lake Superior, 

 can scarcely be said to be settled, though it would seem that they are 

 all older than the fossiliferous Cambrian rocks which practically con- 

 stitute the base of the palaeozoic. I have, I may say, satisfied myself, in 

 regions w T hich I have studied, of the existence and order of these rocks 

 as successive formations, though I would not dogmatize as to the 

 precise relations of those last mentioned, or as to the precise age of 

 some disputed formations which may either be of the age of the older 

 eozoic formations, or may be peculiar kinds of palaeozoic rocks modi- 

 fied by metamorphism. Probably neither of the extreme views now 

 agitated is absolutely correct. 



After what has been said, you will perhaps not be astonished that 

 a great geological battle rages over the old crystalline rocks. By 

 some geologists they are almost entirely explained away, or referred 

 to igneous action or to the alteration of ordinary sediments. Under 

 the treatment of another school, they grow to great series of pre- 

 Cambrian rocks, constituting vast systems of formations, distinguish- 

 able from each other, not by fossils, but by differences of mineral 

 character. I have already indicated the manner in which I believe 

 the dispute will ultimately be settled, and the President of the Geo- 

 logical Section will treat it more fully in his opening address. 



After the solitary appearance of Eozoon in the Laurentian, and of 

 a few uncertain forms in the Huronian and Taconian, we find our- 

 selves in the Cambrian, in the presence of a nearly complete inverte- 

 brate fauna of protozoa, polyps, echinoderms, mollusks, and Crustacea ; 

 and this not confined to one locality merely, but apparently extended 

 simultaneously throughout the ocean. This sudden incoming of ani- 

 mal life, along with the subsequent introduction of successive groups 

 of invertebrates, and finally of vertebrate animals, furnishes one of 

 the greatest of the unsolved problems of geology, which geologists 

 were wont to settle by the supposition of successive creations. In an 

 address delived at the Detroit meeting of the Association in 1875, I 

 endeavored to set forth the facts as to this succession, and the gen- 

 eral principles involved in it, and to show the insufficiency of the theo- 



