230 The American Geologist. October, i892 
period of local elevatiou in mass. But one aad all, mechauical and or- 
ganic alike, they bear in their composition, in their arrangement, and in 
their fossils, abundant and irresistible evidences that they iuere the pro- 
ducts, and that now they are the memorials of the physical geography of 
their time. 
Guided by the principles of Hutton and Lyell, geologists have worked 
out with great care and completeness the effects of those agencies which 
rule in the first of these three life-stages in the history of a mechanical 
formation. No present geological processes are better known to the 
young geologist than those of denudation, erosion, and transportation, so 
familiar to us in the eloquent works of our president. They form to- 
gether the subject matter of that most wonderfully fascinating chapter 
in geology, which, from its modest opening among the quiet Norfolk 
sandhills, sweeps upwards and onwards without a break to its magnificent 
close on the brink of the gorge of the Colorado. But our knowledge 
of the detailed processes of deposition and consolidation which rule in 
the second stage is still exceedingly imperfect, although a flood of light 
has been thrown upon the subject by the brilliant results of the Challen- 
ger expedition. And we are compelled to admit that our knowledge of 
the operations of those agencies which rule in the processes of upheaval 
and depression is as yet almost nil; and what little we have already 
learnt of the effects of those agencies is the prey of hosts of conflicting 
theories that merely serve to annoy and bewilder the working student 
of the science. 
But not one of the formative triad of detrition, deposition, and eleva- 
tion can exist without the others. No detrition is p )ssible without the 
previous upheaval of th^ rock-sheet from which material can be re- 
moved; no deposition is possible without the previous depression of the 
rock-sheet which forms the basin in which the fragmentary material 
can be laid down. 
Our knowledge, therefore, of the origin and meaning of any geologi- 
cal formation whatever, can at most be only fragmentary until this third 
chapter in the life-history of the geological formation has been attacked 
in earnest. 
Now all the rich store of knowledge that we possess respecting the 
first stage in the life of a geological formation has been derived from a 
comparison of certain phenomena which the stratigraphical geologist 
finds in the rock formations of the past, with correspondent phenomena 
which the physical geographer discovers on the surface of the earth of 
the present. And all that we know of the second stage again has been 
obtained in preciselj' the same way. Surely analogy and common sense 
both teach us that all which is likely to be of permanent value to us as 
regards the final stage of elevation and depression must first be sought 
for in the same direction. 
Within the last twenty years or so many interesting and vital discov- 
eries have been made in the stratigraphy of the rock formations, which 
bear largely upon this obscure chapter of elevation and depression. And 
1 propose on this occasion that we try to summarize a few of these new 
