110 P.G.H. boswell—Quantitative Methods in Stratigraphy. 
great regret that tens of thousands of borings should have been 
made and so little of the material met with preserved. For 
mechanical analyses only 10 to 20 grams of the deposit are required, 
and for mineral analyses about a kilogram. It is to be hoped that 
this work will be developed in future. The drawing of sub-formational 
contour-lines has already proved of great value in water-supply 
questions, and in work upon concealed coalfields.' Combined with 
the isopachytes (lines joining points of equal thickness*) of super- 
imposed beds, these contour-lines yield valuable information as to the 
date and trend of folds and faults and the extension of deposits under- 
ground. But it is also necessary to know the lithological variations 
of the buried strata, and if contour-like grade-lines are also to be 
plotted upon maps, and compared with isopachytes and sub- 
formational contours, our basis of work on underground deposits 
must be quantitative. If from the graphical representation of 
sediments (preferably from curves) we can devise a scheme which 
represents by a number the average mechanical composition of 
a rock, we shall, by plotting these numbers referring to ‘ grades’ 
upon a map, and drawing ‘contour-lines’, possess a valuable method 
of indication, not only of thicknesses of beds (by isopachytes), and of 
the form of the surfaces of concealed beds (by contours referred to 
Ordnance datum), but also of changes in lithology, the proximity of 
axes of unrest, etc., in rocks now buried deeply. Such information 
cannot fail to be of considerable economical value, and its academic 
interest is as great. In questions of water-supply much time and 
money will be saved if the thicknesses and exact mechanical composition 
(and therefore the permeability and filtering value) of the various 
members of the overburden with respect to the water-bearing stratum 
are known.? 
Not only the mechanical composition, but the mineral constitution 
also, of each bed occurring in a boring, should be worked out. 
Evidence of unconformity between rock-series is thus accentuated 
and may be of value where fossil evidence is lacking or the relations 
of the beds obscure in the small core of the boring. It may be 
serviceable in certain areas, for example, to regard the change of 
mineral composition which usually exists, as the dividing line between 
the Permian and Triassic systems. The proximity of masses of 
crystalline rocks, of igneous bosses, etc., or even areas of ancient 
sediments forming old land-areas, may be revealed from borings as 
a result of the study of detrital minerals of the sediments bordering 
such land-areas. Additional information may be gained regarding 
1 Since the above was written Professor W. G. Fearnsides has read a paper 
before the British Association upon the Underground Contours of the Barnsley 
Seam (GEOL. MAG., October, 1915, p. 465). See also ‘‘ The Use of Thickness 
Contours in the Valuation of Lenticular Coal Beds’’ (G. S. Rogers and 
C. E. Lesher) : Heon. Geology, vol. ix, p. 707, 1914. 
2 Professor E. Hull used the term isodiametric (or isometric) lines, as those 
joining points of equal thickness of a formation before denudation had acted 
upon it. See Q.J.G.S., vol. xviii, p. 127, 1862. 
* C. S. Slichter, ‘‘ Motions of Underground Water’’?: Water Supply and 
Irrigation Papers, No. 62, U.S.G.S. Hazen’s ‘ Uniformity Coefficient’ obtained 
by sifting is not satisfactory, and is applicable only to coarse deposits. 
