Revieivs — The Origin of Septarian Structure. 519 



silver, lead, zinc, copper, with about 1 dwt. of gold per ton. The 

 distribution and paragenesis of the ores is of interest, since a definite 

 sequence is shown, with copper nearest the surface, followed by 

 galena, blende, and arsenopyrite successively in depth. The whole 

 thickness of the zone series is here unusually small, amounting to 

 only about 500 feet, and it is an interesting subject for speculation 

 as to what metal, if any, may be found at greater depths. The 

 origin of the ores is here referred to an underlying igneous intrusion, 

 probably connected with a dyke of diorite outcropping in the neigh- 

 bourhood of the mine, though it has also been attributed to the 

 influence of the Stanthorpe massif, which, however, appears to be 

 at too great a distance. 



IV. — On the Origin of Septarian Structure. By W. A. Bichardson. 

 Mineralogical Magazine, vol. xviii, pp. 327-38, 1919. 



IN spite of much discussion of the mechanism of septarian structure, 

 the subject has hitherto remained obscure, largely for want of 

 experimental evidence. It is here shown that the common description 

 of radial cracks widening towards the centre is inaccurate and 

 misleading, since the true structure is usually polygonal in section 

 and the width of the cracks is independent of their position in the 

 nodule. A section thus reproduces very closely the appearance of 

 a film of clay allowed to dry on glass. The arrangement of the 

 cracks is compared with the various types seen in drying timber, 

 while experiments were also made with spheres of mud coated with 

 Portland cement and immersed in salt solutions. This process 

 produced a very good imitation of septarian structure, and it. is 

 concluded that the cracking of the nodules is due to chemical 

 desiccation of a colloidal centre, while the nodules themselves 

 originated by the rhythmic precipitation of solutions diffusing 

 through a colloid according to the principles laid down by Liesegang: 

 the material thus precipitated was probably formed by gradual 

 accumulation of bicarbonates by the action of organic by-products on 

 compounds of lime or iron in the rock. It is frequently stated that 

 fossils are commonly found at the centres of septaria, but this occurs 

 with comparative rarity; the majority of those examined contained 

 no fossils at all, and when present they were rarely central. On the 

 other hand many fossils occur in the immediate neighbourhood of 

 barren concretions. 



V. — The Keweenaw Fault. Bv A. C. Lane. Bull. Geol. Soc. 

 Amer., vol. xxvii,"pp. 93-100, 1916. 



THE great fault which forms the southernside of the copper- 

 producing region is of considerable economic and structural 

 interest. In this paper the author shows that it probably began as 

 a block-fault and was also a line of volcanic activity. In the hollow 

 bounded by the fault - scarp the Upper Keweenawan beds were 

 deposited and appear to shade into the Eastern Sandstone of Upper 



