166 THE CANADIAN NATURALIST. [June 



be expected to mingle to some extent with the sub- Arctic type 

 along the Atlantic coast. The Boreal type may be looked for in 

 force on Prince Edward's Island, fringed, as in New Brunswick, 

 by sub- Arctic forms near the shores. In the central and north- 

 western part of Nova Scotia, a partial recurrence of the 

 Continental type may be looked for ; but owing to the moister 

 summers, and nearer proximity to the sea, it is probably more 

 largely mingled with New England forms than it is in the valley 

 of the St. John. 



ON THE PROBABLE SEAT OF VOLCANIC ACTION. 



By T. Sterry Hunt, LL.D., F.E.S. 



The igneous theory of the earth's crust, which supposes it to 

 have been at one time a fused mass, and to still retain in its 

 interior a great degree of heat, is now generally admitted. In 

 order to explain the origin of eruptive rocks, the phenomena of 

 volcanos, and the movements of the earth's crust, all of which 

 are conceived by geologists to depend upon the internal heat of 

 the earth, three principal hypotheses have been put forward. Of 

 these the first supposes that in the cooling of the globe a solid 

 crust of no great thickness was formed, which rests upon the still 

 uncongealed nucleus. The second hypothesis, maintained by 

 Hopkins and by Poulett Scrope, .supposes solidification to have 

 commenced at the centre of the liquid globe, and to have advanced 

 towards the circumference. Before the last portions became 

 solidified, there was produced, it is conceived, a condition of 

 imperfect liquidity, preventing the sinking of the cooled and 

 heavier particles, and giving rise to a superficial crust, from which 

 solidification would proceed downwards. There would thus be 

 enclosed, between the inner and outer solid parts, a portion of 

 uncongealed matter, which, according to Hopkins, may be sup- 

 posed still to retain its liquid condition, and to be the seat of 

 volcanic action, whether existing in isolated reservoirs or subter- 

 ranean lakes; or whether, as suggested by Scrope, forming a 

 continuous sheet surrounding the solid nucleus, whose existence 

 is thus conciliated with the evident facts of a flexible crust, and 

 of liquid ignited matters beneath. 



Hopkins, in the discussion of this question, insisted upon the 

 fact, established by his experiments, that pressure favors the 



