8 University of California Publications in Geology [Vol.8 



CRITERIA APPLIED TO THE BOULDER INTRUSIVE 



Size. — If, now, the distinction between a laccolith and a batholith 

 has been made sufficiently clear, we may turn to the Boulder mass 

 and inquire farther into the evidence upon which it has been placed 

 by all writers in the category of the batholiths. As has been already 

 remarked, no one has as yet attempted to formulate the evidence ; 

 but it seems probable that the size of the mass has been the deter- 

 mining consideration. Weed 14 tells us: "A tract forty miles wide 

 and seventy miles long from north to south, extending from the peaks 

 of Red and Table (Highland) mountains on the south nearly to 

 Helena on the north, and from the Deer Lodge Valley on the west 

 to the Elkhorn Mountains and Bull Mountain on the east, is under- 

 lain by granite, concealed in part by other igneous rocks but constitut- 

 ing one connected mass." Sales 15 describes it as "extending south- 

 westerly from Helena to the Big Hole River in Beaverhead County, a 

 distance of seventy miles. The general outline of the batholith is 

 oblong, but it is extremely irregular in width, averaging about twenty 

 miles." According to Knopf 16 the length of the granite is sixty 

 miles, the average width eighteen miles, and the area about 1100 

 square miles. From these descriptions it is evident that although 

 the mass is sufficiently irregular and indefinite as to its boundaries 

 to lead to differences of estimate, it is nevertheless a very large in- 

 trusive — much larger than the majority of recognized laccoliths. This 

 fact is, however, no bar to the possibility of its being a laccolithic in- 

 jection. The Sudbury laccolithic sheet has an area, according to 

 Coleman, 17 of about five hundred square miles and a volume of six 

 hundred cubic miles ; and this is only an erosional remnant. It is 

 clearly of the same order of dimension as the Boulder intrusive mass 

 with an area of 1100 square miles or more. The Duluth gabbro lac- 

 colith has a length of 120 miles, an average exposed width in its 

 northern part of 22 miles and in its southern part 11 miles. The 

 area of its surface exposure is estimated at 2000 square miles. 18 The 

 great Bush veld laccolith as described by Molengraaff 10 occupies an 

 area of 23,000 square miles. 



U. S. G. S. Prof. Paper 74, p. 26. 

 is Bull. Am. Inst. Min.. Eng., No. 80, Aug., 1913, p. 1526. 

 is U. S. G. S. Bull. 527, p. 29, 1913. 

 it JourjoajLpf Geology, Vol. 15, No. 8, p. 762, 1907. 

 is Van rfise'amL Leith, U. S. G. S. Mon. 52, p. 410, 1911. 

 19 Geology of the Transvaal (Trans, by J. H. Ronaldson, 1904), pp. 42-57. 



