The Origin of the Etruria Marl

Abstract

It has long been known that there lies immediately above the main productive Coal Measures of the Midlands a peculiar group of variegated clays. These beds are known by differ- ent names in the different coalfields, but the best-known term is that of Etruria Marl, first used by Dr. W. Gibson in his elucidation of the sequence in the Upper Coal Measures of North Staffordshire. The position of this group in the Coal Measure sequence is as follows:— A consideration of the nature and distribution of the deposits of Etruria Marl type in the Midlands shows 1.That they are similar in macroscopic and microscopic character and in chemical composition to deposits undoubtedly formed by the decomposition of the basaltic rocks of the Midlands and of many other districts. 2.That they contain, in the vicinity of the Midland basalts, actual fragments of decomposed basalt. 3.That they were deposited at times during which we have proof of extensive denudation of basic volcanic rocks in the Midlands, and in the areas in which we have proof that the products of this denudation were deposited. 4.That no known occurrence of earlier rocks is competent to produce a deposit corresponding in age, character, quantity and distribution to the Etruria Marl of the Midlands. These facts appear to admit of no escape from the conclusion that the Etruria Marl was produced by the decomposition and denudation of the Midland Basalts in