AimiTEESAEY ADDRESS OF THE PRESIDENT. 1x1 



however, is intersected by dykes of a finer-grained dolerite, which 

 has a strong resemblance to the newer dolerite of Grenville, to 

 which an exact date does not appear to have been assigned. 



The observations which are thus placed before ns, and which assign 

 so great an antiquity to the outburst of some of the augitic or pyro- 

 xenic rocks, require us to receive with caution, or at all events to allow 

 exceptions to, the views propounded by continental writers on the 

 age of certain classes of igneous rocks. ITr. David Forbes has re- 

 cently established* the true doleritic character of the so-called white- 

 and-green rock of the Dudley coal-field — a destructive intruder, 

 which must have made its entry, vertically and horizontally, after 

 the consolidation of the Carboniferous strata ; but there is no doubt 

 that in a general way we have to look to still more recent periods for 

 the chief evolution of substances so nearly akin to our modem lavas. 



Certain of the bolder writers of the continental schools make no 

 sort of difficulty in ascribing an igneous origin to many of the 

 crystalline limestones, and especially to those which in many coun- 

 tries are inserted among the gnarled and contorted beds of gneiss 

 and mica-schist. The Canadian surveyors have met with examples 

 of this rock, appearing in a form so like that of true intrusion, 

 filling lines of crack in the gneiss at right angles to the general 

 direction of the strata, that they venture on an explanation differing 

 but Kttle from that of the most thorough plutonist. '' !Many of 

 those rocks," they say, '^ which have hitherto been described as 

 eruptive, are sediments, which, although altered and retaining but 

 little evidence of their former condition, are still in the beds in 

 which they were originally deposited. At the time of their che- 

 mical metamorphism they were impregnated with water, and by 

 the joint action of this and of an elevated temperature were 

 evidently reduced to a more or less plastic mass. This in some 

 cases is found to have been displaced and, having forced its way 

 among disrupted strata, to have assumed the form of an intrusive 

 rock, which, becoming consohdated under sufficient pressure, retains 

 the same mineral character as its parent bed"t. The phenomena 

 observed in some portions of the Laurentian system lead the 

 authors to the conclusion that " the stratified crystalline hmestone 

 of that series was at one time plastic, and when in that condition 

 was forced into the fissures of broken siliceous strata in the vicinity, 

 thus taking the foim of an eruptive rock." 



Another of the crystalline masses which in many regions form an 

 important constituent in the structure of the solid crust of the earth 

 is ophite or serpentine, on which the authors of Canadian geology 

 hold very decided opinions. They assert, indeed, that, although 

 generally described by European geologists as intrusive, the ser- 

 pentines of the Laurentian and some of the overlying formations are 

 " stratified rocks of sedimentary origin, although it is not impro- 

 bable that serpentine may in certain cases, like hmestone, assume 

 the form of an eruptive rock." 



It is noticeable that the serpentines of the Laurentian system 

 * Brit. Assoc. Report, L865. t Geology of Canada, p. 643. 



