308 Lieut.- General C. A. McMahon — 



. . . . The absence of these formations, coupled with the con- 

 glomeratic character of the base of the sandstones, and the inclusion 

 among it of an angular clastic breccia so plainly remade from the 

 in situ brecciated cherty bands of the Infra-Trias, argue an uncon- 

 formity at this spot." Just so ; but what then becomes of the 

 author's argument at pages 135 and 277 (footnote), that the presence 

 of unaltered Tertiary rocks resting in " direct and normal super- 

 position immediately above thin-bedded rocks exhibiting distinct 

 metamorphism " shows that the Tertiaries were deposited later than 

 the metamorphism of the strata, and makes the Tertiary age of the 

 granite " plainly impossible " ? 



If a vast lapse in geological time exists between the metamor- 

 phosed Infra-Trias and the un metamorphosed Tertiaries; and if 

 great thicknesses of beds belonging to the Triassic, the Jurassic, and 

 Cretaceous periods (or some of them) have been removed by erosion, 

 how are we to say what agent caused the metamorphism of the Infra- 

 Triassic strata on which the Tertiary beds now rest ? 



We have j)Ositively no ground for assuming that the metamor- 

 phosing agent in the case of the Laichi Khun beds was granite. 

 Granite does not apparently occur in any part of this section, 

 and the author tells us at page 277 that the metamorphism 

 exhibited by the Infra-Trias " may be attributed to the eruption of 

 basic dykes." The metamorphism of the schistose beds under dis- 

 cussion may have been due to regional metamorphism, to dynamic 

 causes, or to the intrusion of basic igneous rocks, for all we know. 

 What, then, does this section prove regarding the age of the granite ?■ 



If,, for the sake of argument, we were to assume, without a tittle 

 of evidence to support the assumption, that the metamorphism of the 

 thin-bedded rocks was due to the contact action of granite, what 

 would it prove as to the age of the granite ? The fact, it seems to 

 me, would only prove that the granite did not, in this particular 

 section, rise as high as the Tertiary beds ; it would not prove that it 

 was Pre-Tertiary in age. The author tells us (p. 274) that the 

 granite " was formed as great laccolites, deep down below the surface 

 of the earth, under such enormous pressure of the suj)erincumbent 

 rocks that an eruptive function was denied it." That being so, the 

 section relied on, even on the assumption that the metamorphism 

 was caused by granite, would prove no more than that the granite 

 was kept down by the "enormous pressure" of the cover, and did 

 not rise as high as the Tertiary beds. It would not prove that the 

 granite was intruded into the strata where it is now found before the 

 Tertiary beds were deposited at a great height above them. 



The author's third argument (ante) may be divided into three 

 subheads — 



(a) The author has observed no instance of the intrusion of 

 the granite into the " Tanols." 



(h) In the author's opinion the " Tanols " are younger than j 

 the granite. 



(c) In all probability the " Tanols " have derived their material 

 from the granite. 



