THE FOUNDATION-STONES OF THE EARTH. 661 



2,000° F., sufiBcient to melt silver — more than sufficient to melt 

 many lavas — would have been reached at a depth of about four 

 miles. It would now be necessary to descend for at least forty 

 miles in order to arrive at this zone. During the ninety-six years 

 of the century it has been changing its position in the earth's 

 crust, more slowly as time went on, from the one level to the 

 other. There is another consideration. In very early times, as 

 shown by Prof. Darwin and Mr. Davison, the zone in the earth's 

 crust at which lateral thrust ceases and tension begins must 

 have been situated much nearer to the surface than at present. 

 If now, at the end of the century, it is at the depth of five miles, 

 it was at the end of the fourth year at a depth of only one mile. 

 Then, a mass of rock, ten thousand feet below the surface, would 

 be nearly a mile deep in the zone of tension. Possibly this may 

 explain the mineral banding of much of our older granitoid rock, 

 already mentioned, and the coincidence of foliation with what 

 appears to be stratification in the later Archsean schists, as well as 

 the certainly common coincidence of microfoliation with bedding 

 in the oldest indubitable sediments. Pressure, no doubt, has al- 

 ways been a most important factor in the metamorphism of rocks ; 

 but there is, I think, at present some danger in overestimating 

 this, and representing a partial statement of truth as the whole 

 truth. Geology, like many human beings, suffered from convul- 

 sions in its infancy ; now, in its later years, I apprehend an attack 

 of pressure on the brain. The first deposits on the solidified crust 

 of the earth would obviously be igneous. As water condensed, 

 denudation would begin, and stratified deposits, mechanical and 

 chemical, become possible, in addition to detrital volcanic mate- 

 rial. But at that time the crust itself, and even stratified depos- 

 its, would often be kept for a considerable period at a temperature 

 similar to that afterward produced by the invasion of an intru- 

 sive mass. Thus, not only rocks of igneous origin (including vol- 

 canic ashes) would predominate in the lowest foundation-stones, 

 but also secondary changes occur more readily, and even the sedi- 

 ments or precipitates should be greatly metamorphosed. Strains 

 set up by a falling temperature would produce, in masses still 

 plastic, banded structures, which, under the peculiar circum- 

 stances, might occur in rocks now coarsely crystalline. As time 

 went on, true sediments would predominate over extravasated 

 materials, and these would be less and less affected by chemical 

 changes, and would more and more retain their original charac- 

 ter. Thus, we should expect that as we retraced the earth's course 

 through " the corridor of time," we should arrive at rocks which, 

 though crystalline in structure, were evidently in great part sedi- 

 mentary in origin, and should beyond them find rocks of more 

 coarsely crystalline texture and more dubious character, which. 



