MACKIE — THE GEOLOGY OF THE SEA. 



245 



stones), and argillaceous (clays, shales, &c.) ingredients of our 

 earth's crust were generated. 



We are now brought to that age of remote granitoid or gneissic 

 rocks which are the oldest presented to us at this hour of the ancient 

 primeval strata. These, as it is as well to call by a general name, 

 we shall term by their Canadian title of " Laurentian." These are 

 the rocks which constituted the first dry land above the water — at 

 least are the oldest rocks of which any traces remain. 



Now gneiss, in broad terms, may be stated to be regenerated, or 

 at any rate modified granite. Its constituent minerals are the 

 same ; the like three ostensible substances are there — quartz, felspar, 

 and mica. Quartz, one need scarcely say, is one form of silex, or 

 flint ; and mica is a compound of alumina, silica, potash, iron, and 

 fluoric acid. It is, however, the felspar which possesses the chief 

 interest in our present speculations. Felspar has an alkaline base, 

 either soda or potash. Naturally, therefore, felspathic rocks are 

 primarily separated into two sorts — soda-felspar and potash-felspar ; 

 the other alkali, the volatile ammonia, while linking itself with clays 

 and argillaceous earths not entering into any combination, afibrding 

 a felspathic product of the two bases, soda and potash, as they occur 

 in granitic and gneissic rocks ; the former only exists in a soluble 

 state, the potash in its combination producing an insoluhle result. 



No one could describe every purpose and all the phases of even 

 any ordinary object at once, and still more to do so properly we must 

 devote something to other objects to which it may be perhaps even 

 only remotely related, or with which it may be only casually 

 associated. 



if we selected a watch, for example, it would not be sufficient to 

 explain that it was an instrument for measuring time, on the 

 principle of an uncoiling spring checked in its rate of unfolding 

 by a little toothed bar of steel ; the inquiring mind would naturally 

 ask for further explanation, and we should thus be led into mechanics 

 to explain the principles of the action of the mechanism ; into metal- 

 lurgy and jewellery to explain the value and requisites of the 

 materials employed ; we should be carried on to clocks and pendulum - 

 motions ; and finally onwards still to the general history of the 

 methods of measuring the passage of time, from candle-burning and 



