P.G.H. Boswell—Quantitative Methods in Stratigraphy. 105 
that I have urged repeatedly; and the theory may be restated by 
way of a summary of the preceding paper. 
In Loventa forbest (or any other Echinoid), persistent development 
of new plates from the ocular margin causes a downward (orad) 
gliding of the columns. If no structure is developed to interfere 
with this procession, the older plates become gradually resorbed at 
the peristome margin, or all gradually and uniformly decrease in 
size as they increase innumbers. If, however, some special character 
is assumed by a series of plates in a column (whether ambulacral or 
interambulacral), and this character, for reasons of function, must 
be maintained in a definite position in the test (e.g. the large adoral 
plates of the interambulacra in Zovenia or the ambulacral processes of 
the perignathic girdle in a Diademoid), the oncoming plates of the 
column become congested against the barrier thus formed. Under 
such circumstances the plates become lessened in height and restricted 
in width, forming demi-plates, and often coalescing to form compound 
plates. Plate-crushing is first developed, both ontogenetically and 
phylogenetically, at the line of contact between the moving column 
and the barrier. 
The resulting structure may be adapted subsequently for special 
functions (e.g. phyllodes, ‘prehensile’ petals, or consolidation of the 
test fabric), but in its origin it is purely a mechanical outcome of the 
growth of the Echinoid test. 
II].—Tae AppricaTion oF PrrroLosicaL AND QUANTITATIVE MeErHops. 
To STRATIGRAPHY. 
By P. G. H. BosweELu, A.R.C.Se., D.I.C., F.G.S., Imperial College, 
London, S.W. 
I. Inrropuction. 
()* account of the aid it has given, paleontology has been termed 
the handmaiden of stratigraphy, but up to the présent time 
petrology has not been called upon, so far as it might have been, to 
fulfil its appropriate duty towards the elucidation of the problems of 
stratigraphical geology and paleogeography. 
The lithology of sediments has been studied very largely in the field, 
but is still in its qualitative stage. Systematic quantitative work has 
hardly been attempted. Used in conjunction with paleontology the 
broad study has proved of great value in determining facies, and in 
yielding clues to the disposition of land and water and their relative 
changes in past geological times. The fragments contained in the 
coarser rocks, such as breccias, conglomerates, greywackes, grits, 
etc., have been used to some extent petrologically as giving an 
indication of direction and distance of source, but the finer detrital 
minerals of sands, clays, limestones, marls, etc., have not yet been 
adequately studied with the same view. Our ultimate aim must be 
the knowledge of the exact mineral composition of every sedimentary 
rock in the geological column. In such a way only can the economic 
resources of our sediments become familiar, the exact conditions of 
deposit known, and the lesser features of paleogeography realized. 
