4 DE. THOaiAS STEEEY HUNT ON THE 



dykes and in masses of greater or less size, intruded or irrupted among the stratified or 

 indigenous rocks. These are the typical granites of Hutton. The third type includes 

 those concretionary masses of granitic material formed in fissures or cavities, which are 

 evidently deposits from ac[ueous solutions. These are the infiltrated veins of Saussure 

 and of Werner, and are what I have designated endogenous rocks. 



§ Y. By keeping in view this threefold distinction between indigenous, exotic and 

 endogenous granitic aggregates, as I have long since endeavored to show, the obscurities 

 and apparelitly contradictory views of different observers are easily explained. These 

 distinctions are recognized in other crystalline rocks than granite. Under the name of 

 crystalline limestones, as is well known, have been included both indigenous and endoge- 

 nous masses. The question whether or not certain crystalline silicated rocks are to be 

 regarded as eruptive, is seen to be of minor importance, when we consider that it is possible 

 for indigenous crystalline deposits to appear in the relation of exotic masses ; whether dis- 

 placed in a softened and plastic condition, as more generally happens, or else forced, in 

 rigid masses, among softer and more yielding strata, as appears, from the observations 

 of StapfF, to be the case of the serpentines of St. Grothard. * 



§ 8. Werner argued, and as we shall endeavor to show, correctly, from their analogies 

 with concretionary granitic veins, that all granitic rocks were deposited from water, and 

 are consequently indigenous or endogenous in origin. lie denied the existence of exotic 

 and of igneous rocks. Hutton, on the contrary, from the phenomena of exotic granites, 

 and the analogies observed between these and basalts and modern volcanic rocks, was led 

 to assume an igneous and exotic origin for all save the clearly stratiform crystalline rocks. 

 Metalliferous lodes, also, he supposed to have been formed, like granitic A'eins, by igneous 

 injection from below. While the disciples of Werner denied the igneous origin of basalts, 

 and even of obsidian, Hutton and his school, on the other hand, maintained that the agates 

 often found in erupted rocks were formed by fire. Playfair reasons : — " The fluidity of 

 the agate was therefore simple and unassisted by any menstruum ;" that is, it was due to 

 heat, and not to solution ; while, in the case of mineral veins, their closed cavities were held 

 to " afford a demonstration that no chemical solvent was ever included in them." '' These 

 cavities were regarded as due to the contraction consequent on the cooling of injected 

 igneous material. 



§ 9. The basic rocks, included by Hutton under the common names of basalt and whin- 

 stone, are regarded by him as similar in origin to granite, and called " unerupted lavas." 

 He elsewhere says that " whiustone is neither of volcanic nor of aqueous, but certainly of 

 igneovis origin," that is to say plutonic. Playfair distingixishes between what he calls the 

 volcanic and the plutonic theory of basalt. 



But while Hutton ascribed a plutonic origin to basalt and to granite, he did not, as 

 some have done, assign a similar plutonic origin to gneiss and other crystalline schists. 

 These were by Werner declared to result from a continiiation of the same process which 

 gave rise to granite, and to graduate into it. Gneiss is held both by Wernerians and by 

 modern plutonists to be but a stratiform granite, and both of these rocks are believed by 

 the one school to be aqueous and by the other to be igneous in origin. 



* See Trans. Eoj'al Soc. Canada, vol. i, part iv, page 212- 

 ^ Playfair, Illustrations, etc., pp. 79 and 260. 



