REPORT OF THE CHIEF ASTRONOMER 729 



SESSIONAL PAPER No. 25a 



McCoimell long ago noted the remarkable shatter-belt bounding the Trail 

 batholith in the Columbia River valley. (See Sheet 8). Less conspicuous cases 

 occur in many other parts of the Boundary section. In eastern Massachusetts 

 these belts sometimes cover so many square miles together that we must believe 

 that main batholithic masses lie beneath, at but moderate depths. The best 

 published example of a small granite batholith mapped with the distinct pur- 

 pose of illustrating a shatter-zone is doubtless that due to the labours of Coste 

 and White in the Madoc-Marmora Mining District of Ontario.f A reduced 

 copy of this map was published in Volume 16 of the American Journal of 

 Science (1903, page 118). 



TIME RELATIONS. 



The rule that batholithic and stock intrusions to observed levels 

 always follow the climax of orogenic movements is recognized by all geologists 

 who have had wide experience in the study of granites. This systematic time 

 relation seems to hold, with some possible exceptions, from the latest Tertiary 

 back to the date of the youngest pre-Cambrian granites cutting bedded rocks. 

 The ride may not apply to many of the pre-Cambrian batholiths, which seem 

 to have been under severe orogenic pressure during their actual intrusion. 

 Moreover, the greater number of mapped pre-Cambrian, batholiths do not show 

 the same rigour of alignment parallel to distinct orogenic axes as that character- 

 izing the later batholiths. The early pre-Cambrian conditions of intrusion 

 may, therefore, have differed in certain essential ways from the ruling post- 

 Cambrian conditions. 



CHEMICAL RELATIONS. 



Most batholiths are granitic in composition. Some of the largest are com- 

 posed of granodiorite or quartz diorite. A few small batholiths are syenitic. 

 The huge anorthosite masses of eastern Canada, New York State, and Scandin- 

 avia may have true batholithic form and field relations, but this is not certain. 

 ISTo large body of anorthosite of date later than the Silurian is known, while 

 the majority are of pre-Cambrian dates. Stocks have much greater range of 

 composition, including the series from true diorite to aplitic granite, various 

 types of syenite, nephelite syenite, monzonite, etc. 



It is noteworthy that no undoubted batholiths, which are chemically equiva- 

 lent to normal basalt, seem ever to have been mapped. That effusive magma 

 which occurs in the largest quantity, and with such wonderful uniformity of 

 chemical constitution, is not directly represented among the larger subjacent 

 bodies. Even small gabbro stocks are extremely rare, if, indeed, they exist. 



Within the writer's knowledge, no large batholith is known in a petrogra- 

 phic province which does not carry dikes or other injected masses of basaltic 

 composition (diabase, porphyrite, gabbro, or basalt). 



f Geol. and Nat. Hist. Surv. of Canada, Special sheet; \ mile to 1 inch, published 

 without text. 1886. 



