726 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



ing the apophyses, the molar-contact line is usually a flowing one and does not 

 show sharp angles. Whether elliptical or irregular in ground plan, the longer 

 axes of batholiths characteristically run with the average local trends of the 

 respective mountain-axes developed in the orogenic period immediately preceding 

 the intrusions. The batholithic axes may have indefinite relations to axes of 

 earlier and later crustal deformation. In the Cordillera the alignment of the 

 Mesozoic and Tertiary batholiths parallel to the main axis of the chain is 

 evident in the geological maps of both Canada and the United States. 



The downward enlargement of stocks and batholiths to the maximum depths 

 exposed by erosion has already been sufficiently emphasized and illustrated in 

 the description of the Cascade mountains (pages 428 and 494). Many additional 

 examples are figured in Lepsius' 'Geologie von Deutschland/ and in other works. 



The lower limit for the area of exposed batholiths has been arbitrarily fixed 

 at 100 square kilometres, but in very many cases, stocks or bosses are with 

 considerable certainty to be regarded as merely cupola offshoots of large batho- 

 lithic masses, which by continued erosion might be exposed. Indeed, it may be 

 true that every stock and boss is but part of a batholith. The maximum size 

 of pre-Cambrian batholiths may be greater than that of any later one. The 

 batholith of the British Columbia- Alaska Coast range is probably the most 

 extensive of the known post-Cambrian intrusive masses. It is mapped as about 

 1,200 miles long and over 75 miles in average width. One must suspect that 

 this immense terrane is a composite of several, perhaps many, batholiths of 

 different ages. 



That the molar contact of the average stock or batholith cuts across pre- 

 intrusion structures in the country-rock is another obvious fact. This cross- 

 cutting relation is found not only where strong gneisses, schists, and massive 

 rocks compose the country-rock, as at Mount Ascutney, Vermont ;* it is as clearly 

 shown where the Castle Peak stock truncates the soft shales of the Hozomeen 

 range (Page 495.) A multitude of such parallels proves that the shapes of stocks 

 and batholiths are not controlled essentially by variations in the strength of 

 the invaded formations; we have seen that laccoliths are just as regularly 

 located in zones of shales or other rocks more easily split than their respective 

 neighbours. 



The cross-cutting relation of batholiths is sometimes masked, though never 

 annulled, by the development of peripheral schistosity or cleavage parallel to the 

 molar (main) contacts. The best illustration in the Boundary belt is found in 

 the southern contact of the Bayonne batholith, Selkirk range (See map sheet and 

 page 292). Other well known examples occur in the Sierra Nevada,f the Black 

 Hills,:}: and the Rainy Lake region of Ontario. § 



As a rule, batholiths do not develop peripheral cleavage or schistosity in 

 marked degree nor cause important changes in the regional strike of the invaded 

 formations. The large scale, detailed maps of the European surveys are crowded 



• R. A. Daly, Bulletin 209, U.S. Geol. Survey, 1903. 



t H. W. Turner, 17th Ann. Rep. U.S. Geol. Survey, Pt. I, 1895-6, p. 555. 



t C. R. Van Hise, 16th Ann. Rep., U.S. Geol. Survey, Pt. I, 1894-5, pp. 637 and 815. 



§ A. C. Lawson, Ann. Rep., Geol. and Nat. Hist. Survey, Canada, 1887, Part F, map. 



