1 8 WORLD-MAKING 



crust than the material of our old gneisses. As to its bedded 

 character, this may have arisen in part from the addition of 

 cooling layers below, in part from the action of heated water 

 above, and in part from pressure or tension ; while, wherever 

 it cracked or became broken, its interstices would be injected 

 with molten matter from beneath. All this may be conjecture, 

 but it is based on known facts, and is the only probable con- 

 jecture. If correct, it would account for the fact that the 

 gneissic rocks are the lowest and oldest that we reach in every 

 part of the earth. 



In short, the fundamental gneiss of the Lower Laurentian 

 may have been the first rock ever formed ; and in any case it 

 is a rock formed under conditions which have not since re- 

 curred, except locally. It constitutes the first and best example 

 of those chemico-physical, aqueous or aqueo-igneous rocks, 

 so characteristic of the earliest period of the earth's history. 

 Viewed in this way the Lower Laurentian gneiss is probably 

 the oldest kind of rock we shall ever know the limit to our 

 backward progress, beyond which there remains nothing to the 

 geologist except physical hypotheses respecting a cooling incan- 

 descent globe. For the chemical conditions of these primitive 

 rocks, and what is known as to their probable origin, I may 

 refer to the writings of my friends, the late Dr. Sterry Hunt and 

 Dr. J. G. Bonney, to whom we owe so much of what is known 

 of the older crystalline rocks 1 as well as of their literature, and 

 the questions which they raise. My purpose here is to sketch 

 the remarkable difference which we meet as we ascend into the 

 Middle and Upper Laurentian. 



In the next succeeding formation, the middle part of the 

 Laurentian of Logan, the Grenville series of Canada, we meet 

 with a great and significant change. It is true we have still a 

 predominance of gneisses which may have been formed in the 



1 Hunt, "Essays on Chemical Geology"; Bonney, "Addresses to 

 British Association and Geological Society of London." 



