180 Correspondence. 



granitoid diorite may be, especially since he is immediately iaformed 

 that ' it is simply an admixture of hornblende with white and pink 

 felspar.' " For the meaning of granitoid, reference may be made to 

 the glossaries of geological terms. In Mr. Page's handbook it is said 

 to be applied to " such rocks as have the granular-crystalline aspect 

 of granite *' ** without being so in reality." The expression 

 granitoid hornblendic greenstone, however ridiculous it might appear 

 to "the petrologist," ought to be intelligible to anyone acquainted 

 with the literature of geology. 



Mr. Forbes observes that "trap is an extremely vague name to 

 designate rocks by." I have never designated any particular rock 

 by the name of trap, but have used the term in a general way as 

 applied to that great series of igneous rocks which includes many 

 dolerites, melaphyres, basalts, diorites.. etc. 



Again, my critic remarks that I have laid " great stress upon the 

 circumstance that, as instead of being flattened and drawn out, the 

 vesicles found occurring in the rocks are spherical, and are so over 

 considerable areas, the rocks therefore cannot be trappean or igneous." 

 This is an overstatement of what is said. I merely observe that 

 "these appearances, along with other considerations, threw doubt 

 upon the igneous character of the rocks under review." Had this 

 been all the evidence to be gathered it is not likely that I should 

 have regarded it in any other light than as a somewhat anomalous 

 fact, as I had never seen nor heard of so wide an area of amygdaloid 

 destitute of flattened cavities.^ 



It is absurd to say that the development from aqueous strata of cer- 

 tain crystalline rocks (as granite, syenite, hyperite, diallage-rock, and 

 diorite) is a notion supported only by my own assertion. Even those 

 geologists who hold most strenuously by the igneous and eruptive 

 character of all granite must admit with Cotta that the proofs of such an 

 origin are sometimes wanting, and that " there are many circumstances 

 that point to a contrary assumption in certain districts."^ Bischoff has 

 brought forward a vast accummulation of chemical data to show that 

 many of the rocks held by geologists to be of igneous origin may, 

 nevertheless, be due to processes of metamorphism.^ Prof. Keilhau, 



I No one has of late years done more towards the explanation of metamorphic 

 phenomena than the well-known chemist and mineralogist attached to the Geological 

 Survey of Canada. Dr. Sterry Hunt is of opinion " that heated alkaline waters have 

 produced the alteration of sediments," and " that, except in local and comparatively 

 rare cases, the process has only taken place in sediments so deeply buried as to bo 

 directly aflfected by the internal heat of the earth." Whether we agree with him or 

 not in his conclusions as to the causes of metamorphism, those among us who may 

 still cling to the notion that all crystalline rocks which cannot be classed among the 

 gneisses and schists must be of igneous origin, will do well to study the details 

 furnished in the " Reports " of the Canadian Survey. We there find rocks described 

 as metamorphic which at one time would certainly have been coloured upon a geolo- 

 gical map as igneous ; for they frequently present appearances (as, for instance, an 

 amygdaloidal, structure) which are commonly believed to be characteristic of igneoua 

 rocks only. See Geology of Canada (1863), pp. 603, 607. 



* Rocks Classified and Described, p. 388. 



* It is instructive to find this eminent chemist thus endeavouring to "prove" the 

 formation in the wet way of certain crystalline rocks which geologists on the other 

 hand are frequently (not always) well assured must be of igneous and eruptive origin. 



