August 18, 1922] 



SCIENCE 



179 



gard the subsidence as uniform. On the con- 

 trary, it was an unequal warping of the sur- 

 face, which indeed rose and remained land in 

 the region southeast of the canyon, or was but 

 temporarily submerged. 



The invasion of the sea into western North 

 America began with and extended through the 

 so-called Paleozoic age, that vast lapse of time 

 during which life evolved from the grasping 

 crustacean to the ambitious reptile. It is one 

 of the proofs of evolution that although the 

 Paleozoic creatures are long since extinct, their 

 mentalities still persist in individual men. Ac- 

 cording to geologists the Paleozoic was an era 

 which began with the Cambrian period and 

 closed with the Permian. Various intervening 

 periods are distinguished, but for our study of 

 the Colorado River basin, the Paleozoic stands 

 for one event, the advance of ocean waters over 

 much of the continent, their prolonged occu- 

 pation of its area accompanied by numerous 

 changes of front, and their retreat into the 

 permanent ocean basins. 



Back to the beginning of the Paleozoic era, 

 including the Cambrian period, we have fairly 

 complete records of the physical geography of 

 the earth and we can trace the major lines of 

 evolution of organic life. We can even attempt 

 maps of the shifting lands and seas, follow 

 the course of great climatic oscillations, and 

 image in our minds the habitats in which our 

 remoter and nearer ancestors lived. If we draw 

 a parallel between human history and earth 

 history we may compare the dawn of Assyria 

 with the beginning of the Paleozoic. But the 

 remoteness of Assyria is to be measured only 

 in hundreds, whereas that of the early Paleo- 

 zoic is to be estimated in as many millions of 

 years. 



Let us not think, however, that a hundred 

 •million years represents a large proportion of 

 the earth's history, as it is recorded in the 

 rocks of the Grand Canyon. Beneath the 

 earliest Paleozoic strata lie other water laid 

 deposits of sediment, the waste of ancient 

 lands. Only a few fragments of those old 

 records are known, but they testify unmis- 

 takably to the passage of unnumbered ages. 



AVe are prone to think the earth must have 

 been in a different state of cooling or had a 

 different atmosphere in so distant a past. But 



no, the winds blew, rains fell, streams flowed, 

 there was night and day, heat and cold. And 

 within the earth there went on periodically 

 those changes which occasion the rise and sub- 

 sidence of continents, the growth of mountain 

 chains. The Algonkian strata (such is the 

 name geologists use to designate the era) were 

 deeply buried, tilted up, invaded by masses of 

 molten rock and eroded. They record activities 

 identical in kind and intensity with those 

 which are now active in the most youthful 

 ranges, the Rocky Mountains and the Sierra 

 Nevada. Though we look back two hundred 

 million years we find earth-processes the same. 



Even so we have not read the earliest chapter 

 recorded in the rocks of the Grand Canyon. 

 Beneath the Algonkian we come upon an older 

 and different group of rocks. It is a gi-oup 

 which never occurs anywhere but at the bottom. 

 It is the foundation of the superficial crust. 

 I speak of the so-called Archean, the oldest 

 rocks known, though by no means necessarily 

 the oldest rocks ever formed. 



The Archean rocks are not surface rocks, 

 not like the strata of the plateau country. 

 They have risen from depths in the earth's 

 crust where temperatures are high and pres- 

 sures are enormous. The typical Archean are 

 crystalline. Whatever the previous state of 

 the minerals may have been, they have re- 

 crystallized. Some, which are called schists, 

 have i-ecrystallized in a solid state under over- 

 whelming and unequal pressures. They have 

 thus changed form, shortening anad lengthen- 

 ing to fit their Procrustean bed. Others, the 

 granites, have been melted and have intruded 

 as tongues of magma into surrounding masses, 

 causing changes of crystalline form in them. 

 Melting and recrystallizing, crystallizing and 

 remelting, these rocks have undergone changgs 

 so complete that no one can tell what they may 

 once have been nor through what sequence of 

 kneading, mechanical shearing, folding, and 

 chemical changes they may have passed. 



The Arcliean rocks thus represent physical 

 and chemical conditions which exist within the 

 earth's outer shell. I say advisedly exist, not 

 existed. For while it is true that we see only 

 verj' ancient rocks of this character, there is 

 every reason to assume that they are forming 

 now beneath our feet. We know that the 



