Apbil 19, 1907] 



SCIENCE 



621 



face of the Preuss Range in Idaho, at Mont- 

 pelier, Bennington, Georgetown and in the 

 vicinity of Swan Lakes. This discovery has 

 opened a new industry in the west; its future 

 development is dependent on the cost of trans- 

 portation to foreign and domestic markets. 



The 188th meeting of the society was held 

 on February 27 and was devoted to a con- 

 sideration of the 'Methods of Igneous In- 

 trusion.' 



The discussion was opened by Mr. Whit- 

 man Cross, who directed it to the methods by 

 which the large igneous masses, such as 

 laccoliths, stocks and batholiths, have come 

 to the places where they are now visible. 

 The three main agencies called upon in cur- 

 rent literature to account for these masses 

 are: (1) Mechanical displacement of the in- 

 vaded rocks, (2) fusion and assimilation of 

 the rock by the magma, (3) ' magmatic stop- 

 ing.' 



It was claimed that laccoliths, in the sense 

 of Gilbert's original definition, and many 

 closely allied bodies, are beyond question pro- 

 duced by a purely mechanical uplift of rocks, 

 usually sedimentary, above the plane of in- 

 trusion; that assimilation and stoping are at 

 the most rare and subsidiary phenomena. No 

 instances are known to the speaker. 



The origin of stocks and batholiths, viewed 

 as very similar except in point of size, is less 

 evident than that of laccoliths, because we 

 can ascertain the relations for only a portion 

 of each mass. That fusion of country rock 

 by an invading magma, with subsequetat as- 

 similation, is a demonstrated or adequate 

 explanation for stocks and batholiths was 

 denied. This hypothesis is usually advanced 

 with naive disregard for the difficulties in- 

 volved in its acceptance. Among these were 

 mentioned: (1) The manifest impossibility of 

 assimilating and assimilated rocks occupying 

 the same space: (2) the physical problem of 

 supplying and maintaining the heat necessary 

 to keep the magma liquid in spite of conduc- 

 tion into wall rock and absorption in the 

 fusion assumed; and (3) the necessity for 

 demonstrating that an invading magma, as, 

 for instance, one of granitic composition, had 



been changed in character through assimila- 

 tion of quartzite, limestone, or basic igneous 

 rocks. In most stocks and batholiths there is 

 absolutely no evidence that fusion of wall 

 rock has occurred. It can scarcely have taken 

 place on a large scale without leaving evi- 

 dence of such action. While fusion must surely 

 be assumed as taking place under certain con- 

 ditions, there is no good reason to believe that 

 those conditions were realized in known stocks 

 and batholiths. Even should extensive fusion 

 be demonstrated for certain cases, that process^ 

 is not in itself competent to explain the 

 masses under discussion. 



Mr. Cross called special attention to the 

 hypothesis of ' magmatic stoping ' advocated 

 forcibly by Daly in the last few years. After- 

 assiiming that crustal movements must result 

 in liquefaction of rock locally through de- 

 crease of pressure, the magma is pictured by 

 Daly as eating its way upward by a process in 

 which the main factors are the detachment of 

 blocks of rock from the cover of the molten 

 mass, their descent into the lower and hotter 

 parts of the magma and consequent fusion and 

 absorption. It is supposed that the magma 

 may thus quietly rise far into the crust to- 

 horizons which through erosion have in raany- 

 cases become accessible to our observation. 



The magmatic stoping hypothesis of Daly 

 rests upon two fundamental assumptions — 

 viz., that the magmas of stocks and batholiths 

 possess a high degree of liquidity and that the- 

 specific gravity of most crystalline rocks is 

 greater than that of even a gabbroic magma 

 in the assumed liquid condition. The high 

 liquidity of batholithic magmas, although as- 

 sumed by Daly as a matter of common accept- 

 ance, was questioned by the speaker on the 

 basis of recent physical investigations and ob- 

 served facts. In general, the facts of field 

 occurrence are believed to show that the 

 magmas of batholiths have in reality a high 

 sustaining and lifting power; that blocks of 

 country rock do not sink, but rather float, in 

 the magmas; that basic inclusions, often of' 

 considerable size, are brought up from the 

 depths in batholithic magmas. The data at 

 our disposal for estimating differences in 

 density between magmas and solid rocks are- 



