Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/218

 on the other side of the alluvial valley of the river Axe, and which perhaps were originally continuous across the valley and rested mediately or immediately upon the limestone; but which have since been removed by denudating causes, the hardest and most durable part of their mass, the magnesian limestone, being left behind.

I have heard of the following additional cases in which a magnesian breccia is found in connection with red marl. Dr. Wollaston in the first instance and afterwards Mr. Greenough informed me that a similar rock was found near to Cowbridge in South Wales, a specimen of which was presented by the latter to the Society. Mr. Aikin also has noticed a breccia of the same description at Caerdeston and Loton in Shropshire.

In thus comparing the magnesian breccia of Bristol with the yellow limestone of the northern and midland counties, I have assumed that the red marl which lies above the coal measures is of the same order with that which lies at the bases of those escarpments, where strata of mountain limestone are broken off; and where instead of the lower beds rising from beneath the limestone we find horizontal strata of red marl filling the plains. I am not prepared to establish this by any positive proof; such evidence as the geology of the plain of Carlisle would afford is already in the hands of Mr. Buckland; the appearances that are to be sought after for determining this question, and which perhaps may be observed in the neighbourhood of Bristol, are the following: no disposition of the strata is more common in the country between Bristol and the Mendip than that described in Mr. Bright's paper; where a ridge of mountain limestone separates two plains from one another, each containing horizontal beds of red sandstone or marl, the one lying above the limestone