CHAMBERLIN— THE AGE OF THE EARTH. 255 



testimony of this obscured portion as to the horizon at which the 

 sediments began to be deposited, for that is essential to measuring 

 the whole period of deposition. It was once thought that the Cam- 

 brian beds lay close upon "the original crust," and that they either 

 represented the real beginning of the sedimentary series or else an 

 early stage close to the beginning. But as field work progressed it 

 was found that first one and then another thick series of sediments 

 lay below the Cambrian. It was further found that there were 

 marked unconformities between these great terranes, and that these 

 were of such a nature as to imply long intervals of time unrepre- 

 sented by deposits ; that is, times when the deposition took place else- 

 where. The number of such strongly unconformable terranes has 

 been notably increasing as investigation proceeds. The correlation of 

 these is not yet complete or even wholly satisfactory, so far as it has 

 gone, but the leading workers in this field recognize six, eight, or 

 more great stages. This Precambrian factor is thus certainly great, 

 but just how great is yet undetermined. 



The mere extension of the sediments downwards in this large 

 degree is not, however, the most significant feature of this recent 

 work. Great granitic series form a prominent feature of these lower 

 terranes. These were formerly taken to be parts of "the original 

 crust." They have been found, however, to consist of remarkable 

 intrusions into earlier series made up of sediments, volcanic debris, 

 and surface lava flows. The granites are not evidence of " the origi- 

 nal crust of the molten globe." Nor does there seem to be any other 

 trustworthy evidence of "an original crust." Thus observational 

 evidence does not give the depth at which the bottom of the column 

 of sediments is to be found, and theory is perhaps as favorable to a 

 depth of a thousand or two thousand miles as any shallower depth. 

 A reliable starting point for reckoning the total thickness of the sedi- 

 ments is not available. 



The Testimony of the Solutions.^^ 



In the effort to find the earth's age by means of sediments advan- 

 tage may be taken of the fact that each deposit makes its own indi- 



13 Onl}'- a brief general statement could be made at the Symposium for 

 lack of time. Adequate citation of evidence or of authority, or elaboration of 



