30 GROVE KARL GILBERT— DAVIS [Memoirs [ vo™xi i ; 



of conditions as regards rnetamorphism, the Silurian rooks being, usually, merely indurated, and the Archaean 

 invariably highly metamorphic. These two characters of the break serve to show that it represents a vast 

 chasm of time, a chasm, the duration of which may have been greater than that of the ages which have since 

 elapsed. A third character of the break, one that is supported by less evidence, but negatived by none, is that 

 the lowest of the superposed rocks are conglomerates and coarse sandstones. The lowest Paleozoic rocks are 

 Primordial, and the basal portion of the Primordial is everywhere siliceous and of coarse nature. Where the 

 Primordial is absent, and the Carboniferous rests directly on the Archaean, a limestone has been observed at 

 the contact; but this is a local phenomenon, the meaning of which is that certain Archaean mountains were 

 islands in the Silurian sea, and were afterwards covered, or more deeply submerged, by the Carboniferous sea. 

 The conclusion to be drawn from the coarse, fragmental nature of the lower deposits is that the water which 

 spread them was an encroaching ocean, rising to possess land that had long been dry. The recognized interpre- 

 tation of a widespread sandstone is continental submergence, or, what is the same thing, an advancing coast 

 line; and where the formation is important in depth, as well as breadth, we must suspect, at least, that the 

 shore waves sorted, not merely the detritus which they themselves tore from the cliffs of indurated rock, but 

 other debris, which they found already ground, and which needed only redistribution (521, 522). 



It is worth remarking that less mention is made in the second report than in the first of 

 the work done by the advancing ocean in paring down Archean ridges; for although marine 

 erosion was, in 1860 and 1870, the usually accepted method of preparing a flat floor for the 

 reception of unconformable sediments, Gilbert had made much progress, as will be shown 

 later, during his successive seasons in the field, toward recognizing that subaerial degradation 

 could, in areas of weak rocks at least, produce a plain without the aid of marine abrasion. It 

 is therefore significant that the abrasive action of shore waves is limited, in the second report, 

 to their attack on "cliffs of indurated rock." 



SUBMERGENCE OF THE ARCHEAN CONTINENT 



Of still greater importance is a delicately worded protest, next to be quoted, against an 

 old-fashioned teaching that was geologically orthodox in the time of Gilbert's youth — the kind 

 of teaching that Professor Ward pretty surely gave him — to the effect that the Adirondack 

 Mountains in New York and the Laurentian highlands to the north of them represent Archean 

 areas which rose from, not sank into, a wide primordial ocean, and which were as a result sheeted 

 over with a receding succession of strata as the waters withdrew from their emerging flanks. 

 The protest is as follows: 



It would be perhaps, out of place to controvert here the familiar presentation of eastern Paleozoic history 

 as an emergence, beginning with the uplift of the Laurentian highlands, but it may be confidently asserted 

 that western Paleozoic history is the reverse of this. There was a time when the Archaean highlands consti- 

 tuted islands in the Paleozoic sea, but this condition was produced, not by the emergence of these islands, as 

 the nuclei of a growing continent, but by the submergence of the surrounding area, and the consequent abolition 

 of a continent (522) . 



The clearness of insight and the originality of interpretation here revealed are thoroughly 

 characteristic of Gilbert's work; and yet in spite of his convincing demonstration, the orthodox 

 belief which it should have promptly supplanted survived for years; it was still maintained, 

 for example, in so standard a work as Dana's Manual of Geology, not only in the third edition 

 of 1S80, but also in the fourth of 1895. There is probably no better illustration than this one 

 anywhere to be found of the effect of vivid western facts in freeing a philosophic-minded inquirer 

 from domination by orthodox eastern theory. As with other generalizations, to be mentioned 

 later, this one seems to have been based less upon observations in the Great Basin than in the 

 plateaus; for the smaller and scattered localities of Paleozoic basal unconformity found in the 

 basin ranges are much less impressive than the majestic continuity of its exposure deep in the 

 Grand Canyon. There truly the magnificent display is writ so plain that he must indeed run 

 far who readeth it. Yet this novel and fundamentally important interpretation of a sinking 

 Archean continent, a geological discovery of the first rank, does not seem to have impressed 

 itself deeply upon Gilbert's modest mind; for when, only 10 years afterwards, a correspondent 

 who did not have the above-quoted passage in mind, wrote to him asking if he had not some- 

 where expressed an opinion about the pre-Paleozoic submergence instead of emergence of the 

 Adirondacks as an outlier of the Laurentian highlands, the reply was, " There is some mistake 

 in regard to my opinion about the Adirondacks, for I have none. I have never seen them." 



