Previous to the year 1913 the English Kimmeridge Clay was divided into two zones only, which were believed tq be “intimately blended”. In 1913 Dr. H. Salfeld instituted a more detailed subdivision of eleven zones, the upper three of which constitute the Upper Kimmeridge Clay as since defined by Dr. F. L. Kitchin. Salfeld's sequence of zones was emended in 1922 by Messrs. Chatwin and Pringle, who showed that at Swindon the zone of Pectinatites pectinatus (Phill.) occurs below the Swindon Clay, and not above it, as Salfeld had supposed. In the early part of last year Mr. S. S. Buckman also presented a similar scheme of zones, and subdivided Salfeld's zone of P. pectinatus, which he placed in the Lower Portland, as in Salfeld's original scheme.

In the following pages I have endeavoured to give an interpretation, not only of the appearance of ice ages, but also of the greater changes of climate registered in the geological record. In view of the importance ascribed in this discussion to relief, I have named the theory the relief hypothesis.

During the course of a recent excursion of the Cambridge Sedgwick Club to the Coniston District, under the guidance of Professor J. E. Marr, important evidence was obtained upon the relationship of the Coniston Limestone Series to the Borrowdale Volcanic Series. A visit was paid to the area south-west of Torver Beck, when, after Professor Marr had pointed out the marked difference between the strike of the Coniston Limestone and that of the underlying Borrowdale rocks at High Pike Haw, the actual strongly discordant junction was located by Professor O. T. Jones and the writer. This was found to be one of unconformity and not a fault, and was traced by the party for some distance.

The diversity in mineral composition of metamorphic rocks can be regarded as the resultant of two independent variables, first of the ultimate composition of the rocks, and secondly of the grade of metamorphism, or the particular pressure-temperature conditions under which the rocks have arisen. It is clear that these two variables may well form the basis of a classification of rocks of metamorphic origin.