468 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



Scotland, could be formed under the influence of the various forces 

 here at work. While great areas of the earth's crust must have 

 been dislocated in this manner, it is quite possible that other great 

 areas may have been able to preserve, throughout these convul- 

 sions, their originally horizontal position. That part of the fluid 

 material which may have protruded itself through the fractured 

 crust, we may reasonably imagine to have solidified somewhat out 

 of the range of the internal currents, and to have produced the 

 first erupted granites. We may further reasonably suppose that 

 the same fluid material must have penetrated into the interstices 

 between the various fragments of the original crust, and have 

 solidified there. This fractured and re-consolidated part of the 

 crust would then present exactly the same appearance, so far as 

 the relations of the various rocks are concerned, as the primitive 

 strata of Canada, Scandinavia, and the north of Scotland do at the 

 present day. That is to say the strata of gneiss, granite, mica and 

 hornblende schist would be arranged in highly inclined positions) 

 and if in contact with rocks of later periods, the latter would over- 

 lie the primitive strata unconformably. This peculiar build of the 

 primitive rocks is characteristic of those districts where they have 

 been admitted to be the oldest rocks on the earth's surface. Thus, 

 in some of the Western Islands of Scotland, the primitive strata 

 are not overlaid by any newer rock ; and in Canada the vertical 

 strata of the Laurentian series are in many places covered by the 

 horizontal beds of the Potsdam sandstone. In Norway the out- 

 crops of the highly inclined primary strata frequently occupy areas 

 of several hundred square miles, and, at the outskirts of these areas 

 they are overlaid by fossiliferous strata of the Silurian' system. The 

 primitive rocks of Brazil, consisting of gneiss, gneiss-granites, 

 granite, syenite, mica-schist, and hornblende rocks, extend north 

 and south through fourteen degrees of latitude, and have a breadth 

 of 250 geographical miles, in which enormous area, strata inclined 

 from 45 to 70 degrees are alone observable. We cannot suppose 

 that these rocks were originally formed in this position ; nor can 

 we reasonably regard them as a system of strata, having, in their 

 original horizontal position, a thickness of upwards of 100 geogra- 

 phical miles. There remains only the explanation given above, 

 that the originally not very thick strata assumed their highly in- 

 clined position owing to the lateral pressure to which they were 

 expose! ; the latter having been caused, partly by the contraction ex- 

 perienced by the globe in cooling, and partly by the protrusion of 



