THE FOUNDATION-STONES OF THE EARTH. 653 



aid in interpreting the cryptogram which, shrouds its history, or 

 must we reply tliat there is neither voice nor language, and thus 

 accept with blind submission, or spurn with no less blind incre- 

 dulity, the conclusions of the physicist and the chemist ? The 

 secret of the earth's hot youth has doubtless been well kept ; so 

 well that we have often been tempted to guess idly rather than to 

 labor patiently. Nevertheless, we are beginning, as I believe, to 

 feel firm ground after long walking through a region of quick- 

 sands ; we are laying hold of principles of interpretation, the rela- 

 tive value of which we can not in all cases as yet fully apprehend 

 — principles which sometimes even appear to be in conflict, but 

 which will some day lead us to the truth. The name Cambrian 

 has been given to the oldest rocks in which fossils have been 

 found. This group forms the first chapter in the first volume, 

 called Pal£eozoic, of the history of living creatures. Any older 

 rocks are provisionally termed Archsean. These — I speak at 

 present of those indubitably underlying the Cambrian — exhibit 

 marked differences one from another. Some are indubitably the 

 detritus of other, and often of older, materials — slates and grits, 

 volcanic dust and ashes, even lava-flows. Such rocks differ but 

 little from the basement beds of the Cambrian ; probably they are 

 not much older, comparatively speaking. But in some places we 

 find in a like position rocks as to the origin of which it is more 

 difficult to decide. Often in their general aspect they resemble 

 sedimentary deposits, but they seldom retain any distinct indica- 

 tions of their original fragmental constituents. They have been 

 metamorphosed, the old structures have been obliterated, new 

 minerals have been developed, and these exhibit that peculiar 

 orientation, that rudely parallel arrangement which is called foli- 

 ation. That these rocks are older than the Cambrian can often 

 be demonstrated. Sometimes it can even be proved that their 

 present distinctive character had been assumed before the over- 

 lying Cambrian rocks were deposited. Such rocks, then, we may 

 confidently bring forward as types of the earth's foundation- 

 stones. I must assume what I believe few, if any, competent 

 workers will deny, that certain structures are distinctive of rocks 

 which have solidified from a state of fusion under this or that 

 environment ; others are distinctive of sedimentary rocks ; others, 

 again, whatever may be their significance, belong to rocks of the 

 so-called metamorphic group. Our initial difficulty is to find 

 examples of the oldest rocks in which the original structures are 

 still unmodified. Commonly they are like palimpsests, where the 

 primitive character can only be discerned, at best faintly, under 

 the more recent inscription. Here, then, is one of the best which 

 I possess — a Laurentian gneiss from Canada. Its structure is 

 characteristic of the whole group ; the crystals of mica or horn- 



