Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/284

 nate of lime associated with carbonate of magnesia, forming a pasty cement by which the rounded boulders and pebbles constituting the deposit are firmly held together ; it is not to be confounded with the Permian magnesian limestones of the N.E. of England. The proportion of lime and magnesia appears to be indefinite in the uncrystallized dolomite, which is a mere mixture of true dolomite and carbonate of lime.

Messrs. Buckland and Conybeare were the first to apply the term " dolomite " both to this formation and to the carboniferous limestone when dolomitized in situ, or so-called dolomitized limestone.

In 1817 Mr. Warburton * had previously stated with reference to the conglomerate and associated New Red Sandstone in the neighbourhood of Bristol and the Mendip Hills, that " if denudatory or other disturbing causes were in action previously to the deposition of the red marl, we might expect to find the red marl immediately incumbent upon any rock from the coal-measures to the granite inclusive."

Buckland and Conybeare in 1822† showed that the older and disturbed rocks of the Bristol district were not only covered unconformably by various beds of the. New Bed Sandstone series, but also that higher formations, such as the Lias and Oolite, were brought into contact with them in the same relative position.

Messrs. Riley and Stuchbury subsequently had occasion to notice these beds under peculiar and interesting circumstances, announcing at the same time the discovery of two genera of reptilia in this conglomerate on Durdham Down, near Bristol ; these they respectively named Thecodontosaurus and Paloeosaurus. From that time to the present no occasion has arisen to call attention to these apparently uninteresting rock-masses, which, however, locally are of much importance.

I should, however, here mention that my valued friend, W. Sanders, Esq., F.R.S., of Clifton, during the construction of his large geological map‡ of the Bristol coal-field was so impressed with the belief that the Dolomitic Conglomerate occupied in some places the lowest part, and in others the middle (in time) of the Keuper series, that he purposely omitted inserting the conglomerate as a separate formation upon Ms map, but incorporated or massed it with the New Bed Sandstones and marls, calling the whole the Keuper marls and sandstones. His map therefore fails to show the geographical position of the conglomerate on the higher or more elevated tracts of land surrounding the Bristol coal-field. That he was right in associating it with and placing it at the base of the New Red Sandstones and marls cannot be doubted ; and the numerous sections and conditions under which I have examined it, both in the

with the so-called magnesian limestone of Gloucestershire and Somersetshire (which forms part of the dolomitic conglomerate), or with the conglomerate itself.

† Trans. Geol. Soc. 2nd ser. vol. i. p. 217, &c. ‡ Constructed upon the scale of 4 inches to the mile.
 * Trans. Geol. Soc. vol. iv. p. 209 : 1817.