522 Reports & Proceedings — Inst. Mining and Metallurgy. 



and junction terraces, was not clear as to the mode of formation of 

 what he himself called " the lateral terrace proper ". Under that 

 head he seemed to include (1) terraces with straight or slightly 

 curved, nearly parallel front and back margins, and (2) terraces with 

 strongly curved back margins concave towards the river. Professor 

 Davis, elaborating an idea little more than suggested by Miller, 

 propounded in his paper on " The Eiver Terraces of New England " a 

 satisfactory theory, not so much of terrace-formation as of terrace- 

 ■preservation. "The lateral terraces proper" of Miller, particularly 

 (2) above— the most difficult of all to account for — still required 

 explanation. Such terraces had evidently been swept out in the first 

 instance by pronounced and fairly persistent bends, to which the 

 characteristic curves of their back margins was due. In the case of the 

 Dee, the river was not withdrawn by short-circuiting across the necks 

 of the spurs within the bends ; for the slope of such terraces was, on 

 the whole, towards, not away from the present course of the river. 

 The conditions determining the formation of river islands and their 

 incorporation in the flood-plain were discussed in detail. It was 

 pointed out that every terrace of any extent along the Dee was 

 traversed by a larger or smaller number of anastomosing hollows, 

 plainly former channels of the river or of branches of it ; and that 

 terraces of all types had been built up by the accretion of island to 

 island through the desertion and subsequent partial obliteration 

 (by flood-loam, etc.) of secondary channels. The process was shown 

 to have illustrations elsewhere ; and the principle involved was no 

 doubt of wide application in the case of rivers of fairly steep gradient 

 whose flood-plain, like that of the Dee, Avas practically coextensive 

 with their meander belt. 



3. "A Rhomb Porphyry Boulder from the Glacial Deposits at 

 Bay of Nigg, Aberdeen." By Dr. Alex. Bremner, Aberdeen. 



The Rhomb Porphyry — a boulder weighing 321b. — was found 

 15 feet above H.W.M.O.S.T. in a boulder-bed, far too coarse to be 

 called a gravel, underlying the Lower Grey Boulder Clay of the 

 Aberdeen district (see Trans. Edin. Geol. Soc, vol. x, pt. iii, 

 p. 345). The probable ways in which it might have reached the 

 l^lace where it was found were discussed, and the resemblance 

 between the glacial sequence at Bay of Nigg and that on the York- 

 shire coast was pointed out. This was the first record of the 

 occurrence of a Scandinavian boulder, in situ in glacial deposits, 

 on the mainland of Scotland. 



Ia^stitution op Mining and Metallurgy. 

 October 21, 1920. 



" The Origin of Primarv Ore Deposits." Bv J. Morrow Campbell, 

 D.Sc, F.G.S. 



The author commences at the period when the outer silicate shell 

 of the earth was molten. The primeval magma is regarded as having 



