408 proceedings: geological society 



("pipes") had been loci of rich minerahzation. There was reason to 

 suppose that a large bathoHth underlay the region, from which, Paige 

 believed, the ore-minerals had been distilled. S. R. Capps inquired 

 regarding the field criteria for distinguishing between the several forms 

 of intrusive bodies; laccoliths, apically truncated stocks, and medially 

 truncated stocks. Butler replied that this would be difficult in some 

 cases, but in the region he had described the stocks show distinct cross- 

 cutting characteristics, while the laccoliths had produced evidences of 

 doming in the associated sedimentaries. The boundary between api- 

 cally and medially truncated stocks is somewhat arbitrary, l)ut often 

 the age of the associated formations gives a basis for judgment as to 

 the depth to which erosion has been carried. For example, if the wall- 

 rocks are of pre-Cambrian strata, erosion is believed to have reached 

 a great depth in the stock. A. C. Spencer thought Butler's ideas 

 regarding association of minerals with character of stock and depth 

 of truncation very valuable. He believed that analyses of small, 

 undifferentiated sills or laccoliths for ore minerals should give basis of 

 estimation of the original content of the magma in such minerals, before 

 the differentiation which had been effective in larger bodies had resulted 

 in concentration in some parts and impoverishment in others. F. L. 

 Ransome was inclined to differ with Spencer's opinion on this matter 

 and doubted if such analyses would be of value in drawing conclusions. 

 J. B. Umpleby referred to the Idaho districts with which he was 

 familiar. They did not seem to conform with Butler's ideas regarding 

 relation between mineral deposition and depth of truncation. J. T. 

 SiNGEWALD, Jr., referred to the tin deposits of the Erzgebirge. There 

 was a similar association there to what Butler had described. Tin 

 veins were found around the apices of granitic stocks, while more 

 deeply eroded stocks showed no accompaniment of veins. N. L. Bowen 

 brought up the question of the probable degree of solidification sup- 

 posed to have been reached in the stocks at the time of ore-deposition. 

 If the apices had reached a condition of complete solidification there 

 was no apparent reason why they should serve as channels for ore- 

 bearing solutions. Umpleby thought that in such cases solidification 

 had not l)een complete at time of ore-deposition. 



Bailey Willis: Physiographic provinces of South America. The 

 speaker described in some detail the broader physiographic features 

 of the continent, such as the delta of the Amazon, the chain of the 

 Andes, the pre-Andean depression, and the plains of Argentina, and 

 explained the processes to which their origin and present condition 

 are due. In many places over broad areas very recent movements 

 of depression or elevation have occurred and may still be continuing. 



Discussion: Keith inquired whether the Argentine plain which had 

 been described was simple or whether it was made up of two or three 

 or several plains which were more or less distinct. Willis replied that 

 in the broad expanse from the pre-Andean depression to the Atlantic 

 there is no evidence of faulting, but warping has produced irregularities 

 of surface. C. H. Wegemann inquired as to conditions of sedimenta- 



