GEOLOGICAL EXPLORATIONS AND LITERATURE. 61 



basic material, as lime, iron, vtv., lioriil)lcinlc and allied miiicral.s were formed, giving 

 rise to syeiiitic loeks. Where these basic elements existed iu still larger ijroportions, 

 aud the silica was relatively less abundant, horiiblendie and allied rocks were formed, 

 and in similar wnys other variations in the constitution of the sediments gave rise to 

 other variations in the crystalline results. 



The changes were not carried so far, in most cases, as to destroy all traces of the 

 original bedding of the vsediment, or to mix the material of adjacent layers iu any nota- 

 ble measure. There are certain massive portions, however, in which nearly all dis- 

 tinct traces of original sedimentation are obliterated. 



To conceive in detail of the exact method by which these remarkable transfor- 

 mations took place, lays a heavy tax upon the scientific imagination, and certainly 

 transcends the limits of demonstrable science. In general terms, however, the meta 

 morphism may (luite safely be said to be due to combined chemical and molecular 

 forces, acting under the conditions of (1) pressure, (2) heat, and (3) moisture. Beyond 

 reasonable doubt the strata in question x>resented these conditions, while undergoing 

 the distortions already described (pp. 7i-75). 



Igncoun phenomena of the Laureniian. — The Laurcutian rocks are frequently 

 traversed by dikes, veins, or irregular masses of intruded rock. These are most 

 commonly composed of granite, but are sometimes of the darker basic classes. It has 

 not been determined how far the phenomena may be due to ti'ue igneous penetration 

 from below, and how far to the rendering of the rock of certain portions of the series 

 suQicien.tly plastic by heat and moisture to be forced into cracks aud fissures of 

 adjacent portions. In either case the essential nature of the action was the same, the 

 difterence being in degree of liquefaction and the soui'ce of material (pp. 77-78). 



(2) As to a separation between Lanrentian and Hiironian : 



We have said that Laurentian sedimentation drew to a close, but it was only 

 because the elevatory forces just described forced the beds up from the ocean, and 

 l)revented further accumulation upon them. But sedimentation elsewhere did not 

 cease. The wash of the land, the wear of the waves, and the settling of silts beneath the 

 sea continued ceaselessly. Even while the great elevation was in progress, the land 

 was being worn and beds were accumulating in the adjacent sea, and as soon as it 

 reached its loftiest height it began at once to be cut down aud carried back to the sea 

 by the agency of the great leveler, water. 



Of the sediments formed during the elevation and immediately after — for a time 

 whose limits are yet unknown — we know nothing. They are deeply buried from sight 

 iu our region, and if their equivalents elsewhere have been seen they have not yet 

 been determined to be such. So far, therefore, as the details of the history are con- 

 cerned, it is an unrevealed chajjter. The lecord is not destroyed, as are certain pages 

 of human history, but it has not yet been reached aud read (p. 78). 



