52 
ON - GEOLOGICAL PHOTOGRAPHY. 
near the so-called “ copper-mills,” would not only show them, by 
the presence of layers of flints, to he in the Upper Chalk, but 
would also give an instructive lesson in physical geology, for we 
should find that the chalk where covered by gravel is permeated 
by “ pipes,” but that where covered by clay there is not a “ pipe ” 
to be seen, the gravel allowing water charged with carbonic acid 
to percolate through and dissolve the chalk, and the clay forming 
an impervious capping. This is also well shown in the new 
cuttings on the extension of the Metropolitan Railway between 
Rickmansworth and Chorley Wood. A photograph of the chalk¬ 
pit on Reed Hill, near Royston, where an anticlinal may be seen 
in the Chalk, would show that earth-movements of considerable 
magnitude have occurred since the deposition of the Chalk, and 
that a range of hills may sometimes coincide with an anticlinal, 
though here the connection is no doubt accidental. 
Very instructive would be photographs of sections of the 
Woolwich and Reading Beds and London Clay exposed in our 
brick-fields near Watford and elsewhere, for they would show the 
variable character of the estuarine and fluviatile Reading Beds, 
and the persistent character of the marine London Clay. Even 
many of our gravel-pits might be photographed with advantage to 
show the varying characteristics of gravels of different ages and 
situated at different heights. 
But the photographs of most general interest would doubtless 
be those illustrative of the physiographic features of the country; 
for instance a series showing the Chalk escarpment in its entire 
range through the north of Hertfordshire and the adjoining counties. 
Such a series would show the influence upon our scenery of the 
Totternhoe Stone and the Chalk Rock and of the varying hardness 
of the different beds of the Chalk, proving the configuration of the 
country to be almost entirely dependent on the action of sub-aerial 
denudation on beds which withstand it in varying degrees. 
Almost any views taking in a considerable extent of country 
may indeed be rendered of geological interest by accompanying 
them with diagrams indicating the formations represented. 
Enough has perhaps been said to show that although there is 
but a small series of rocks present in Hertfordshire, there is suffi¬ 
cient of geological interest to give, with the aid of the photographer, 
an additional zest to our field meetings, to enrich the collections 
of our Society with an interesting series of views, and to advance 
the science of geology, especially in its educational aspects, in 
which pictorial representation is such a powerful aid. 
