There have been in the past few years opportunities, which are not likely to recur, for collecting fresh and unweathered samples of the Upper and Lower members of the LondonTertiary series in the neighbourhood of Harrow. It therefore seemed to be worth while making a series of mechanical analyses of these samples for comparison with analyses which have been made of samples from the same formations in the other parts of the London basin.

The correlation of isolated outcrops of igneous rocks where two or more similar intrusions are exposed is a difficult problem which is not always solved by thin sections or field studies. Such a problem was encountered in mapping the closely related Pikes Peak and Silver Plume granites of pre-Cambrian age in the Sawatch Range of central Colorado (Fig. 1). A comparison of the heavy minerals of the isolated outcrops with those of known granites was undertaken; and for this purpose large samples, suitable for crushing and heavy mineral analysis, were collected from various points within the areas of each batholith, and from the small outcrops whose age was in question. It was hoped that sufficient similarities in the heavy mineral assemblages might be established to be of value in making correlations. Furthermore, as work on the heavy minerals in igneous rocks is still in the experimental stage, a series of analyses from various parts of a given batholith should throw some light on the question of whether heavy minerals may be distinct and constant enough to be characteristic and so give a reliable means of correlation of isolated exposures.

During the last twenty years the Pleistocene of Central Europe was investigated by a great number of geologists. As a result it is now possible to establish a detailed stratigraphy which proves to be generally applicable. When employed in conjunction with certain astronomical results it enables horizons, fossils, and implements of many localitiesto be dated in thousands of years.

Consideration of the continental areas over which the sea spread at various times during the course of geological history leads to the impression that perhaps every part of the land surface of the earth was, at one time or another, raised from a position below sea-level. The present land surface seems to have grown around ancient nuclei owing to the compression of the granitic, sedimentary, and other rocks of which the continental layers are composed. At certain times the compressive forces acted so strongly upon these rocks that the average height of the land reached maximum values, which might have been as great as 3,000 feet. Then denudation, and perhaps other processes, coming into operation and persisting through ages of comparative quiescence, lowered the surface, sometimes to minimum average levels, which might have been as low as 500 feet. There have been a few comparatively short periods of exceptionally high relief, separated by longer periods of low relief. Assuming that the land was raised by horizontal compression, the diminution in surface area of the earth necessary to produce a change in level of 2,500 feet would be very considerable. Instead of considering the change from a minimum average height of 500 feet to a maximum of 3,000 feet, it may be more convenient in the first place to consider the elevation of, say, the continental layers of Dr. H. Jeffreys from sea-level to the average height of the present land surface, that is about 2,500 feet, or 762 metres. It will be supposed, then, that the continental layers, as they now exist, were developed from layers consisting of 1 km. of sediments of specific gravity 2·4, 10 km. of granite of specific gravity 2·6, and 20 km. of tachylyte of specific gravity 2·9, the whole resting on dunite of specific gravity 3·3.