THE FOUNDATION-STONES OF THE EARTH. 6^7 



eral in crystallizing could, as it were, elbow out of tlie way the 

 more refractory particles. Its effects are, in brief, to consolidate 

 the rock, and, while causing some constituents to vanish, to in- 

 crease greatly the size of all the others. It follows, then, that 

 mineral segregation is promoted by the maintenance for some 

 time of a high temperature, which is almost a truism. I may add 

 to this that, though rocks modified by contact -metamorphism 

 differ from the Archsean schists, we find in them the best imita- 

 tions of stratification-foliation, and of other structures character- 

 istic of the latter. One other group of facts requires notice before 

 we proceed to draw our inferences from the preceding. Very com- 

 monly, when a stratified mass rests upon considerably older rocks, 

 the lower part of the former is full of fragments of the latter. 

 Let us restrict ourselves to basement beds of the Cambrian and 

 Ordovician — the first two chapters in the stone-book of life. What 

 can we learn from the material of its pages ? They tell us that 

 granitoid rocks, crystalline schists of various kinds, as well as 

 quartzites and phyllites, then abounded in the world. The Torri- 

 don sandstone of Scotland proves that much of the subjacent 

 Hebridean had even then acquired its present characteristics. 

 The Cambrian rocks of North and South Wales repeat the story, 

 notably near Llynfaelog in Anglesey, where the adjacent gneiss- 

 oid rocks from where the pebbles were derived, even if once 

 true granites, had assumed their present differences before the 

 end of the Cambrian. By the same time similar changes had 

 affected the crystalline rocks of the Malverns and parts of Shrop- 

 shire. It would be easy to quote other instances, but these may 

 suffice. I will only add that the frequent abundance of slightly 

 altered rocks in these conglomerates and grits seems significant. 

 Such rocks seem to have been more widely distributed— less local 

 —than they have been in later periods. Another curious piece of 

 evidence points the same way. In North America, as is well 

 known, there is a great group of rocks to which Sir W. Logan 

 gave the name of Huronian, because it was most typically devel- 

 oped in the vicinity of Lake Huron. Gradually great confusion 

 arose as to what this term really designated. But now, thanks to 

 our fellow-workers on the other side of the Atlantic, the fogs, gen- 

 erated in the laboratory, are being dispelled by the light of micro- 

 scopic research and the fresh air of the field. We now know that 

 the Huronian group in no case consists of very highly altered 

 rocks, though some of its members are rather more changed than 

 is usual with the British Cambrians, than which they are sup- 

 posed to be slightly older. Conglomerates are not rare in the Hu- 

 ronian. Some of these consist of granitoid fragments in a quartz- 

 ose matrix. We can not doubt that the rock was once a pebbly 

 sandstone. Still, the matrix, when examined with the microscope, 



TOL. XXXIT. 42 



