622 THE LOWER SILURIAN PERIOD. 



St Lawrence, and the movements of its beds may have begun in con- 

 nexion with the disturbances which Logan and others have shown to 

 have occurred at the close of the Lower Silurian period; but the 

 intrusive granite appears to be continuous with that of Devonian age 

 described in a previous chapter. 



Whatever view may be taken of the age of the granitic rocks 

 of this group, it is certain that they are strictly hypogene rocks, 

 that is, that they belong to the deep-seated foci of subterranean heat, 

 and are not superficial products of volcanic action. They are sub- 

 stances such as we might expect to find, could we penetrate miles 

 below the surface, beneath modern volcanoes. They were therefore 

 probably at one time buried deeply, and have been brought up by 

 movements of dislocation, and by the removal of their superficial 

 portions by aqueous agents. They have without doubt furnished 

 much of the material that has been employed in building up the 

 more recent formations of the country. 



This leads to the question, Can we discover in the subsequent 

 rock formations evidences of such an origin, and can the changes 

 which these derived materials have undergone be satisfactorily ex- 

 plained ? This subject, the genealogy of rocks as it may be termed, 

 is of some interest, and I may glance at it in its bearing on the 

 geology of Nova Scotia. 



The granite of Nova Scotia and its associated gneiss and mica- 

 slates are among the oldest rocks found in the province, and we 

 may therefore take them and their derived rocks for illustrations. 

 The products of the decomposition of granite are quartz sand, scales 

 of mica, and fine clay which results from the decomposition of felspar. 

 Such materials, when washed down and deposited in water, will 

 form coarse and fine sandstones, micaceous sandstones and flags, 

 arenaceous and argillaceous shales ; and these may, by heat and 

 pressure, be converted into quartzite, mica slate, and clay slate. 

 From pure white granite the derived detritus would be colourless 

 or nearly so. But the mica and felspar of many granites contain 

 iron, and the sulphuret of iron is also present in some granites. In 

 these cases the derived sediment will have a yellow or buff colour, 

 from the presence of the yellow oxide of iron ; or in some cases 

 the clay may have a red colour, from the peroxide of iron present 

 in red felspar. Of course, when the granites contain hornblende 

 or are syenitic, much more iron may be present in the derived 

 sediment. In nature nearly all soils of granitic origin are more 

 or less coloured in these ways. In this manner, buff, brown, and 

 red clays, and buff and brown sandstones may be produced. 







