Recent research on the polymorphism of calcium orthosilicate has broadened and to some extent modified the picture of the phase relations of this substance as presented in the pioneer studies of Day and Shepherd (1906) and Rankin and Wright (1915). These investigators recognized three distinct forms, α, β, and γ, related by transformation as follows: α⇄β at 1420° C. and β⇄γ, at 675° C. The high-temperature α form was described as monoclinic or triclinic and intimately twinned, the β form orthorhombic, and the low-temperature γ form as probably monoclinic. The optical properties of these α and β forms as reported are so closely similar that were it not for the twinning, regarded as diagnostic, the two forms would be distinguishable only with difficulty. This optical similarity was later found to be paralleled by a close agreement of the X-ray powder diffraction patterns of the two phases early noted by Hansen (1928) and confirmed among others by Insley (1936).

In the progressive thermal metamorphism of siliceous dolomites, tremolite and calcite are usually considered the earliest of the new mineral phases formed by reaction of the primary materials, dolomite and quartz. An early formation of forsterite has also been recorded, as, for example, in the dolomitie zones of the Durness limestone surrounding the Tertiary Beinn an Dubhaich granite of Skye. In this aureole, however, the relation of forsterite to tremolite has hitherto not been closely investigated. As Bowen has pointed out in his systematic study of the progressive metamorphism of siliceous limestones and dolomites, a first formation of forsterite from dolomite and quartz could not rank as an equilibrium reaction since forsterite and quartz can have no stable coexistence. Tremolite has accordingly been regarded as the first metamorphic product in an equilibrium series of reactions of increasing decarbonation affecting siliceous dolomites in an advancing higher temperature environment at any given pressure.

The important influence that the radioactive decay of the radio-elements uranium and thorium has had on the geochemical distribution of lead is now well appreciated, but little is known about the possible geochemical implications of the radioactivity of rubidium.

Like its alkali metal homologue potassium, rubidium is β radioactive and slowly disintegrates into a stable isotope of strontium: small quantities of strontium have therefore been accumulating in various minerals since the earth's crust first consolidated. This paper will describe firstly, the abundance of radiogenic strontium in the earth's crust as a whole, and secondly, the distribution of radiogenic strontium in various minerals and rocks, in particular those which are of igneous origin.

Measurements with the petrological microscope may be divided into those made with the orthoscopic and those with the conoscopic arrangements. It was early realized that if the micrometer scale or compensator was inserted in the focal plane of the ocular, the same apparatus would serve for measurements of both kinds, provided that a Bertrand lens was available. The method was described by A. B. Dick as early as 1888 in a form of microscope which has remained popular with many workers up to the present time. Additional slots at the level of the focal plane of the ocular were cut through the mount and the upper part of the microscope tube. Most European manufacturers, however, continued to provide only the usual slots in the lower part of the tube.