168 THE CATADIAN NATURALIST. |Yol. ix. 



ground which he explored enables nie to Jiffinii that no one will 

 ever be able permanently to overset the general le.iding sub- 

 divisions which he established in the Laurentian and Huronian 

 systems. 



Let us turn now to the particular points brought before us in 

 the papers to which reference has been made. It may be well 

 however first to notice some general geological facts which must 

 be present to our minds if we would enter intelligently into these 

 discussions. The formations with which we have to deal in 

 the more ancient geological periods all belong to the bed of the 

 sea. Now in the sea bottom there have been in process of depo- 

 sition, side by side and contemporaneous!}', four diiferent kinds 

 of material, differinii- extremelv in their mineral character and in 

 the changes of which they are susceptible. Tlie first of these 

 consists of earthy and fragmental matter washed by water from 

 the surface or sea margins of the land and deposited in belts 

 along coast-lines, or on broader areas where ocean currents have 

 been drifting the detritus sround from the land by ice or washed 

 down by great rivers. The second consists of organic remains 

 of shells, corals and foraminifera. accumulated in coral reefs and 

 the debris washed from them, in shell beds and in the chalky 

 ooze of the deep ocean. Some beds of this kind are very widely 

 distributed. The third is composed of material ejected by igne- 

 ous action from the interior of the earth and either spread in the 

 manner of lava-flows or of beds of fragments and fine volcanic 

 ash. Such rocks naturally occur in the vicinity of volcanic 

 orifices, which are often disposed in long lines along coasts or 

 crossing ocean basins, but fragmental volcanic matter is often 

 very widely spread b}^ ocean currents and is interstratified with 

 other kinds of aqueous deposit. The fourth and last description 

 of bedded matter is that which is deposited in a crystalline 

 form from solution in water. In later geological times at least, 

 such deposits take place in exceptional circumstances, not of fre. 

 quent occurrence. Such beds are dolomite, greensand, gypsum, 

 and rock salt. 



Now it may be afiirmed that at each and every period of the 

 earth's geological history, all or most of these kinds of deposit 

 were in progress locally. But it may also be afB.rmed that in 

 certain geological periods there w^as a predominance of one or 

 more over very great areas; and that in any particular area, even 

 of considerable size, there may be definite alternations of these 

 different kinds of material characteristic of particular periods. 



