SOME UNSOLVED PROBLEMS IN GEOLOGY. 829 



by layer, in the manner of aqueous deposits. On the other hand, its 

 chemical composition is quite different from that of the muds, sands, 

 and gravels usually deposited from water. Their special characters are 

 caused by the fact that they have resulted from the slow decay of 

 rocks like these gneisses, under the operation of carbonic acid and 

 water, whereby the alkaline matter and the more soluble part of the 

 silica have been washed away, leaving a residue mainly silicious and 

 aluminous. Such more modern rocks tell of dry land subjected to at- 

 mospheric decay and rain-wash. If they have any direct relation to 

 the old gneisses, they are their grandchildren, not their parents. On 

 the contrary, the oldest gneisses show no pebbles, or sand, or lime- 

 stone nothing to indicate that there was then any land undergoing 

 atmospheric waste, or shores with sand and gravel. For all that we 

 know to the contrary, these old gneisses may have been deposited in 

 a shoreless sea, holding in solution or suspension merely what it could 

 derive from a submerged crust recently cooled from a state of fusion, 

 still thin, and exuding here and there through its fissures heated waters 

 and volcanic products. 



It is scarcely necessary to say that I have no confidence in the sup- 

 position of unlike composition of the earth's mass on different sides, 

 on which Dana has partly based his theory of the origin of continents. 

 The most probable conception seems to be that of Lyell ; namely, a 

 molten mass, uniform except in so far as denser material might exist 

 toward its center, and a crust at first approximately even and homo- 

 geneous, and subsequently thrown into great bendings upward and 

 downward. This question has recently been ably discussed by Mr. 

 Crosby in the London " Geological Magazine." * 



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 recurred, except locally. It 

 constitutes the first and best example of these chemico-physical, aque- 

 ous 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, incandes- 

 cent globe. For the chemical conditions of these primitive rocks, and 

 what is known as to their probable origin, I must refer you to my 

 friend Dr. Sterry Hunt, to whom we owe so much of what is known 

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

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

 able difference which we meet as we ascend into the middle and upper 

 Laurentian. 



In the next succeeding formation, the true lower Laurentian of 

 Logan, the Grenville series of Canada, we meet with a great and sig- 

 * June, 1883. f Hunt, " Essays on Chemical Geology." 



