Origin of certain Granites, j^c. 55 



met with. If intended in a chemical sense, nothing short of actual 

 analysis conld prove their presence. 



Again (page 521, Quart. Journ.), the crystallographer •will be 

 rather puzzled by the reference to "Porphyritic Felspar Crystals." 



(3) Petrology. — In such an investigation as the present, it ought 

 to be superfluous to insist upon the most scrupulous care and atten- 

 tion being devoted to the petrological character and description of 

 the rocks forming the subject of the inquiry ; for how otherwise can 

 geologists know that they are referring to one and the same rock in 

 their inquiries ? 



Here it is to be feared that the G-eological Survey have not shown 

 an example worthy of imitation, when such names are used as 

 syenitic granite for horablendic granite, augitic greenstone for 

 dolerite, greenstone porphyry for porphyritic greenstone, along with 

 granitic porjjhyry, syenitic porphyry, dioritic porphyry, trachytic 

 porphyry, felspar porphyry, syenitic greenstone, felspathic green- 

 stone, felspathic trap, etc., etc. ; and when upon examination in the 

 field, rocks coloured as greenstones on the map of the Survey fre- 

 quently turn out to be dolerites, felstones, altered clay slates, etc., 

 whilst at the same time no explanations have been furnished by the 

 Survey, whether mineralogical or chemical, for the use, or rather 

 misuse, of such names. 



Surely the petrologist may throw up his hands in despair when 

 he finds Mr. James Geikie defining minette as a quartzless granite ; 

 just as soon would he expect to see limestone defined as a clayless 

 marlstone, or as a calcareous sandstone without the sand. If the 

 mineral component which specially characterises granite, when com- 

 posed with all other analogous rocks, is to be left out, surely there 

 can be no sense in retaining the name of granite at all. 



Such similes are unworthy of the geologist as tending to mislead 

 others less versed in petrology ; for even if minette was proved to 

 have been formed from greywacke,' by hydrothermal or any other 

 action, such a fact does not value one iota in proving that it also 

 could be transmuted into granite, but the reverse. 



At page 527, Quart. Journ., petrologists are told, " that under the 

 term dioritic are included all those rocks which consist essentially 

 of silicates of lime and magnesia set in a Felspathic base or matrix," a 

 definition which no petrologist would for a moment admit, for then 

 he would have to regard as diorites, not only many porphyrites, 

 dolerites, &c., but also such true volcanic lavas of the present period 

 as are composed of a felspar with olivine, wollastonite, monticellite, 

 augite, etc. 



The petrologist defines diorite to be a rock composed of one or 

 more felspars with hornblende, and regards greenstone as that 

 variety of diorite in which green or dark-coloured hornblende either 

 predominates, or, when the rock is fine grained, renders more 

 obscure the presence of the felspar. 



What therefore a "granitoid diorite," (referred to page 530, GtEOl. 



^ A rock which, it must be remembered^ consists essentially of seventy-five per cent, 

 of quartz. 



