GEOPHYSICAL LABORATORY. 155 



of the enormous deposits of calcium carbonate present in the crust of 

 the earth, just as its local increase is responsible for the characteristic 

 features of a limestone country. 



Again, calcium carbonate exists in two well-known crystalline forms, 

 calcite and aragonite, the latter relatively rare; indeed, this was the 

 first kliown example of this phenomenon of polymorphism. The rela- 

 tion between these two has been somewhat uncertain, but it is now 

 established that at atmospheric pressure and at all temperatures above 

 0° aragonite is unstable with respect to calcite and tends to go over 

 into the latter. The rate of the process of transformation depends 

 upon a number of factors, not all yet understood; but under the con- 

 ditions prevailing in the earth's crust it is very slow. The appearance 

 of aragonite constitutes another cause of the genesis and persistence 

 in time of an essentially unstable form in which the equilibrium is 

 only apparent; a state of things which requires further investigation 

 before general statements with respect to such cases are made. 



COPPER-SULPHIDE ORES. 



The study of the secondary enrichment of the copper-sulphide ores 

 which was undertaken in collaboration with Professor L. C. Graton, of 

 Harvard University, was reported at some length a year ago. Pro- 

 fessor Graton has carried out an elaborate plan of field investigation 

 of the occurrence and environment of these ores, while this laboratory 

 has been pursuing a systematic study of the chemical and physico- 

 chemical relations between the different sulphides and the possible 

 effects of percolating waters upon them. Detailed reports upon vari- 

 ous phases of the investigation are in preparation. 



It will be recalled that a primary copper-sulphide ore consists of a 

 mass of pyrite or pyrrhotite, in which is embedded a much smaller 

 quantity of the sulphides of copper, copper-iron, etc. Above such an 

 ore-body there is commonly a capping or gossan of oxide of iron of 

 varying thickness, between which and the primary ore is a secondary 

 band of especially rich ore which consists for the most part of chalcocite 

 or cuprous sulphide mixed with smaller quantities of other copper 

 minerals. The primary ore below usually shows no sign of alteration 

 since its original deposition. 



These and other facts of field observation have led to the belief: 



(1) that such ore-bodies consisted originally of primary ore only; 



(2) that oxidation through the action of moist air occurred upon its 

 exposed surface; (3) that the sulphates formed by this oxidation were 

 dissolved by meteoric water and percolated downward; (4) that cop- 

 per was precipitated from these solutions by the yet unchanged sul- 

 phides, thus forming the enriched ore, while the iron became limonite 

 and formed the capping or gossan. 



At the outset the purpose of this inquiry was formulated thus: 

 (1) what are the essential conditions under which any of the important 



