TRANSACTIONS OF SECTION C. 507 
acid shows that the marl from this locality is highly calcareous; and thin 
sections of it under the microscope reveal the condition in which some, if not all, 
of the calcareous matter exists. Scattered through the matrix of the rock there 
occur great numbers of minute crystals of rhombohedral carbonates. Experiments 
made by the author and Mr. Russell F. Gwinnell upon some of these crystals 
isolated from the rock proved that they are not calcite. Further tests by micro- 
chemical methods demonstrated the presence in them of both calcium and mag- 
nesium, and pointed to their being dolomite To test this identification a washed 
residue of the marl, containing practically nothing but quartz grains and the 
crystals in question, was submitted to Mr. G. S. Blake, of the Imperial Institute, 
for analysis, with the result that the identification was confirmed. 
These dolomite crystals, which have the form of the fundamental rhombo- 
hedron, and are extraordinarily perfect, are always very minute, ranging in size 
from individuals just visible to the naked eye to others which can only be seen 
with high powers of the microscope. They occur in the red parts of the marl as 
well as in the green. They were also found in specimens of the marl from other 
localities in the district, extending as far north as Worcester, and doubtless occur 
in it outside the area already investigated. Moreover, they occur sometimes in 
great profusion ; indeed, from analyses by Dr. Moody! of samples from the Wain- 
lode exposure, it may be inferred that they sometimes make up as much as 25 per 
cent. of the mass. 
Very similar variegated marls occur in the Gloucester area, at the base of the 
Old Red sandstone, and samples of these were compared with the Keuper marl. 
It is an interesting and probably significant fact, in connection with the conditions 
of formation of these two deposits, that no dolomite crystals were found in these 
Devonian marls. 
It seems likely that the presence of these crystals in the Keuper mail is an 
expression of the special conditions which prevailed during its accumulation ; and 
the author, though aware of other possible modes of origin, is inclined to the view 
that they were precipitated from solution from the waters of an inland sea or 
a lake at the same time as the remainder of the marl was being deposited from 
suspension. This view is supported by the fact that other substances thrown down 
from solution during the desiccation of large sheets of water, such as gypsum and 
rock salt, are familiar constituents of the marl of other and not distant localities. 
Tt has been suggested that the Keuper marl may represent an accumulation cf 
wind-borne dust, a Triassic loess formation. But the presence of these dolomite 
crystals in it seems to the author to militate against this view and to point to the 
conclusion that such portions of it, at any rate, as contain the crystals were laid 
down under water. 
The existence of these dolomite crystals is interesting from one other point 
of view, inasmuch as it affords a possible explanation of the fact that in many 
localities where the New Red marls rest directly upon limestones these latter are 
found to be partly or wholly dolomitised. Waters obtained from the Keuper 
marl at Blaisdon, near Gloucester, have recently been shown to be so highly mag- 
nesian as to be unfit for drinking; the magnesium has presumably been acquired 
by the solution of the minute dolomite crystals. Such waters by percolation 
through underlying limestones might very well effect the dolomitisation so often 
observed in them. 
MONDAY, AUGUST 5. 
The following Papers and Reports were read :— 
1. Iron Ore Supplies. 
(i) By Beynert H. Broven, F.G.S. 
Of all the problems with which the practical geologist has to deal, none is of 
greater importance at the present time than the discovery of fresh sources of iron 
* Quar. Journ, Geol. Soc., vol. \xi. 1905, p. 431. 
