GEOPHYSICAL LABORATORY. 139 



limonites, has almost no magnetic effect. We have here, in the 

 magnetic siisceptibihty, a criterion of origin to add to the ferrous iron 

 percentage. 



Although ferric oxide has commonly a very low magnetic suscepti- 

 bility, yet it can be obtained with a susceptibility of the same order of 

 magnitude as that of magnetite itself by the slow oxidation of magnetite 

 at low temperatures. One natural example of this ''true magnetic 

 hematite" (as distinguished from the common magnetic hematite 

 whose magnetism is due to ferrous iron) has been found. This peculiar 

 persistence of magnetic properties in spite of a radical change in the 

 chemical composition points to some fundamental atomic origin for the 

 phenomena of magnetism, which can perhaps be approached through 

 the study of these oxides. 



There is still another magnetic property about which we know 

 practically nothing in the natural oxides of iron, namely, polarization 

 or permanent magnetization. Natural magnets of varying strength 

 are frequently found in magnetite deposits, although the bulk of such 

 deposits are entirely unpolarized. These differences shov/ that differ- 

 ent conditions of origin and history have undoubtedly existed, whose 

 elucidation waits upon physical and chemical study of the artificial 

 oxides in the laboratory. 



A beginning has been made upon the polycomponent systems 

 containing ferric oxide as one component, in the investigation of the 

 system CaO-Fe203. Only two compounds occur in the system: 

 2CaO.Fe203 and CaO.Fe203. Both dissociate before melting; the 

 former at 1436° and the latter at 1216°. The eutectic between Fe203 

 and Ca0.Fe203 melts at 1203°. All mixtures between about 15 and 25 

 weight per cent CaO, therefore, melt within a few degrees of 1200°. 

 The properties of this system are of interest in connection with port- 

 land cement, in which a small proportion of AI2O3 is frequently replaced 

 by Fe203; and also as a partial basis for the study of various metal- 

 lurgical slags. 



Progress has also been made in the characterization of the different 

 crystalline hydrated ferric oxides, in connection with the work on 

 secondary enrichment of sulphide ores. 



CALCIUM CARBONATE. 



Calcium carbonate is without doubt the most important individual 

 constituent of the sedimentary deposits, and its mode of deposition 

 has been discussed, primarily or incidentally, in a very large number 

 of papers. Nevertheless, despite the attention bestowed upon this 

 topic, there remain inconsistencies and gaps in our knowledge of it. 

 In the consideration in this laboratory of the deposition of calcium 

 carbonate, to which reference was made in the annual report for 1915 



