168 P.G. H. Boswell—Quantitative Methods in Stratigraphy. 
Oscillatory and alternating current action has an effect, not only 
in eliminating the more easily decomposable minerals (e.g. some of 
the ferromagnesian minerals, apatite, andalusite, etc.), but also in 
concentrating the heavy detritus by a kind of natural jigging or 
panning. ‘ Pay-streaks’ may thus be produced. Parts of the pebble- 
bed under the London Clay in Suffolk exhibit an enrichment in 
zircon, rutile, ilmenite, and magnetite by this action, especially when 
there is evidence of current-action and contemporaneous erosion. 
The lenticles of grains of titaniferous hematite occurring in the 
basal Cambrian conglomerates of St. Non’s Bay, near St. David’s,! 
may have a similar origin. The richness of the Bagshot Beds in the 
heaviest minerals may be attributed to a like cause. 
As opposed to this concentration, we have the elimination of 
minerals (other than those which are more or less unstable) by 
conditions of deposit. The work of Thoulet and Retgers has shown 
that micas are absent from certain wind-formed deposits such as 
desert and dune sands, but examination of British blown-sand dunes 
has revealed their presence. Topaz tends to form flakes on account 
of its good basal cleavage, and may well float away; it has been 
suggested that andalusite may be lost in this way, by floating off 
during natural or artificial panning. The scarcity of apatite in many 
deposits is probably due to its disappearance by solution. It is more 
abundant in clays and loams than in sandstones where percolation is 
easy, but it does also occur in the latter. When the sediment has 
to be cleaned by long boiling with acid, much apatite undoubtedly 
dissolves, but, as in the case of the Boxstone Bed, some still remains. 
Professor C. G. Cullis and Professor W. G. Fearnsides have both 
suggested that the difference in solubility may be due to the presence 
of either or both chlor- or fluor-apatite, but usually the grains are 
so small, and the occurrence so uncommon, that micro-chemical 
proof is difficult. 
What may be called the intensive study of an area of sedimentary 
‘rocks of various geological ages, surrounding an igneous or meta- 
morphic complex, is at present very necessary for the advancement 
of our work on the petrology of sediments. For this purpose we 
need to know exactly and in detail the mineral composition of 
neighbouring crystalline rocks, slicing alone not being sufficient. 
Crushing must be resorted to, and panning and separation with 
heavy liquids, in order that we may be certain of knowing all the 
accessory and rarer minerals and their relative abundance.? Following 
this, the study of the neighbouring sediments of varying geological 
age, considered stratigraphically, cannot fail to yield valuable data 
which will serve to indicate the extent to which the differences 
observed in assemblages of detrital minerals may be caused by 
direction of drainage, conditions of deposition, stability of minerals 
in geological time, and contemporaneous or subsequent sealing of 
minerals in sedimentary rocks. 
1 'W. Jones, Proc. Geol. Assoc., vol. xxii, p. 232, 1911. 
2 R. H. Rastall & W. H. Wilcockson, Proc. Geol. Soc., p. xxx, 1915. 
