474 Notices of Memoirs — A. R. Hunt — Devonshire Schists. 



of sinking to overcome the velocity of the stream, and only begins 

 to sink on the retardation of that velocity. The detritus with any 

 particular rate of sinking will thus descend faster and also be 

 crowded into a narrower area at a distance from the shore, and so 

 form a thicker deposit there, where alone there is permanently 

 room for it. This result will be modified by (1) the expansion 

 laterally of the retarded stream, (2) the evaporation of the surface 

 water, (3) the mixture with sea-water, (4) the superposition of 

 various maxima at different distances from shore, (5) the redistri- 

 bution by tides and currents. 



During the continuance of constant physical conditions the 

 seaward boundary of river-brought deposits will thus be a marked 

 line — the ' mud-line ' of Dr. Murray — at various depths, according 

 to circumstances. At this line there is a rapid change of slope. 

 This has been called an escarpment, and the edge of the continental 

 plateau, but it is suggested that it is really the limit of terrigenous 

 deposits in bulk. 



The lateral expansion of the stream divides it into two spirals 

 on the two sides of the axis and separated by it. Along this axis 

 the deposit will be carried farther to sea than on the two sides. 

 This will cause an apparent depression of the sea-bottom opposite 

 the mouths of direct rivers. These depressions have been taken 

 to be submerged river-channels, but they are the natural result 

 of the form of the deposit, except in special cases — of which the 

 Congo may be one. Such depressions may be seen indicated on 

 charts of the sea opposite suitable rivers, as on the west coast of 

 Spain and India, and on the east coast of America. 



Original organically formed limestones require not only water 

 free from sediment, which may be found between the openings 

 of large rivers, but an abundant supply of the organisms producing 

 them. It is seen from the results of the Challenger expedition that 

 60 per cent, of the species of such animals, and probably a higher 

 one of the specimens, inhabit the first 100 fathoms, and another 

 20 per cent, the next 100 fathoms. Limestones are, therefore, most 

 likely to form narrow lenticles with the long axis parallel to the 

 shore, as in the case of barrier reefs ; and when we find them giving 

 place to shales, it is not because we are approaching a shore-line, 

 but because in going parallel to the shore-line we are approaching 

 a source of sediment. 



VI. — The Evidence of the Htdeothermal Metamokphism of 

 THE Schists of South Devon. By A. E. Hunt, M.A., F.G.S.' 



THE author contends that a dominant cause of alteration in the 

 Devonshire schists was the presence of water ; in other words, 

 that as the presence of some degree of heat is not disputed the 

 metamorphism was hydrothermal. He divides the schists into 

 two groups : — 



1. Eocks which have been variously called chlorite- schists, horn- 

 blende-schists, epidote-schists — or, generally, green rocks. 



1 Abstract of a paper read before the British Association, Belfast, Sept., 1902, ia 

 Section C (Geology). 



