Walter Keeping—The Upware and Potton Pebble-beds. 419 
A glance at the map will show that the Upware and Potton beds 
rest upon members of the Upper and Middle Oolites, and most of the 
Jurassic pebbles, phosphatic or otherwise, are doubtless of local 
origin. But some of the Portlandian masses with Cyrena (?) rugosa, 
etc., are decidedly southern, identical with the Swindon Portlandians ; 
and a particular set of shells, including Pholadomya tumida, Ag., and 
Myoconcha porilandica, Blake, are remarkable as occurring also as 
derived fossils in a band of the Portlandian group at Swindon.! 
Bunter beds, capable of furnishing pebbles such as many of our 
quartzites, and likely to have been exposed at that time, lie roughly 
between two lines drawn N. and N.W. from Potton. The hard 
sandstone pebbles g, are likely to be from the Coal-measures, and 
many of the remainder are probably of Silurian, Cambrian, and pre- 
Cambrian ages. 
The set of fossils in the pale slaty pebble k, points to the Bala or 
the May Hill group; the general appearance of the rock being 
nearest to the Shropshire Caradocs ; but it is more perfectly cleaved 
than the Onny river-beds. This occurrence of Lower Paleozoic fossils 
is also of value as supporting evidence of the nature of the indurated 
argillites or Lydian stones h, many of which closely resemble the 
more earthy varieties of dark chert. Many of the quartzites c, and 
the rhyolite n, are probably of similar primeval date. We have 
already seen that the chert was derived from both the Carboniferous 
and Jurassic periods. ' 
In reasoning from these materials concerning the details of their 
accumulation, it is of the first importance to determine whether the 
pebbles were obtained directly from the parent rock in Neocomian 
times, or only indirectly from earlier pebble beds; and, again, we 
must consider wether they were carried to their present positions 
solely by marine currents, or had been brought down from the land 
by rivers. Some of the quartzites are like those of the New Red 
Sandstone pebble beds, and were probably thus derived, but the 
majority of the rocks are not similar to those of common occurrence 
in the older conglomerates. In connexion with the abundant 
angular chert fragments, a specially noteworthy fact is the absence 
of any trace of the mother-limestone itself; and yet the Mountain 
Limestone is a very durable rock under wave action, and there is 
nothing in the Upware deposit to hinder the preservation of any 
included calcareous fragments. 
Perhaps the best evidence of the direct origin of some of the older 
pebbles is the fragment of fossiliferous Lower Paleozoic rock, whose 
very angular contour excludes any theory of repeated transport. 
The general physical features of the Neocomian period are pretty 
well known, and an outline of some of these facts will here be of 
service to us in discovering the source of the more ancient pebbles. 
1 See Blake on the Portlandian rocks of England, read before the Geological 
Society, January, 1880. 
2 It is indeed remarkable that in the New Red Sandstones we know of no such 
accumulations of chert pebbles, derived from the Mountain Limestone, to correspond 
with the abundant flint pebble and gravel beds of Tertiary and modern times. There 
must surely be some such deposits somewhere hidden in the New Red. 
