How Rocks are Formed 159 



crust. The other principal elements which enter into its composition 

 are carbon, sulphur, hydrogen, chlorine, phosphorus and fluorine. All 

 these are styled metalloids while among the metals are aluminum, 

 calcium, magnesium, potassium, sodium, iron, manganese and barium. 



Having thus secured a solid rock floor, of which we have most 

 excellent illustrations in the range of Laurentian hills to the north of 

 the river Ottawa, great portions of which presumably represent some of 

 our first formed rock, the next development presumably was the precipita- 

 tion of water, through the chemical union of the oxygen and hydrogen 

 which entered largely into the composition of the gaseous envelope 

 surrounding the newly created earth. From the geologist's standpoint, 

 this may almost be regarded as our next rock formation ; for throughout 

 the whole subsequent history of the earth's devlopment, down to the 

 present day, water has played a very important part. Gradually the 

 watery envelope increased till, possibly, it swept resistless around the 

 entire globe. By degrees, through the cooling and shrinking of the 

 crust, ridges would be produced which formed barriers against which 

 the mighty waves beat with the terrible force of the primeval ocean 

 surge, tearing down and grinding to powder the newly formed coast 

 line, and in this way the conditions were furnished by which the great 

 thickness of the sedimentary formations which form so instructive a field 

 of study to the working geologist, was laid down. 



Taking this then as our starting point in geological time we may 

 say that the greater part of the subsequent formations, as we now know 

 them, was produced through the agency of fire or water. By the first 

 we mean that certain portions of the earth's crust have been brought to 

 the surface by means of volcanoes or great fissures in its surface, through 

 which the liquified interior rock issued. These rocks are therefore 

 known as igneous or volcanic, and are styled intrusive when the liquid 

 has solidified before reaching the surface as in the case of granites, 

 syenites &c, or eruptive when the intrusive matter has cooled or 

 hardened after reaching the surface. Among these latter are the 

 diorites, traps and volcanic ejectamenta generally. There is however 

 a manifest difference in the character and composition of these two 

 groups, the latter being ofen darker hued and finer grained, the 



