In the following pages an account is given of the rock assemblages produced at the contact of a dolcrite intrusion with chalk. Owing to the intense localized assimilation of calcium carbonate by the dolerite magma, unusual rock types are produced. The products of the endogenous contact-zone are dealt with in detail. Those of the exogenous contact-zone have in part been described already, but some additional data are incorporated in this account.

Since W. Lindgren first detected the presence of microphenocrysts of analcime in basalt, many observers have recorded this mineral as a constituent of igneous rocks, but there is some difficulty in distinguishing between analcime and glass and between analcime and leucite in thin rock-sections. For instance, H. Rosenbusch identified the interstitial colourless isotropic material in monchiquites as glass, whereas L. V. Pirsson showed it to be analcime. H.S. Washington showed that basalts from Sardinia, previously thought to be leueitic, contain phenocrysts of analcime. A. Lacroix also found, in his study of north African basaltic lavas, that in many cases so-called leucite-lavas actually contain analcime and not leucite.

As grunerite is a somewhat uncommon mineral, and as its occurrence at Pierrefitte in association with lead and zinc ore is unusual, if not unique, it may be worth while to put on record the following notes on some material which I collected during an examination of the Pierrefitte deposits.

The name 'grunerite' was originally given to a fibrous mineral approximating in composition to FeSiO3 and occurring with magnetite and garnet at Collobrières in the dept. of Var, in the south of France, which was described by Gruner in 1847. It was originally supposed to be a pyroxene, but was later shown by Des Cloizeaux and Lacroix to belong to the amphibole group. A similar mineral occurs in the iron deposits of the Lake Superior district, and elsewhere ; but it has not been recorded from the Pyrenees.

The material on which the following description is based was presented by Dr. A. L. Hall to the Department of Mineralogy at Cambridge, and I am greatly indebted to him and to Prof. A. Hutchinson for affording me the opportunity of describing it. The first account of the occurrence was given by Dr. Hall, recording from Aarkop, Magoloring, 'a beautiful delicately rose-coloured mica … in compact dark bluish-grey ore as thick, short, colmnnar aggregates not unlike little books of mica with pseudo-hexagonal outlines and showing the characteristic perfect basal cleavage … the mate: rial was found on the higher slopes defining the west side of Martha's Kloof'. This description was amplified somewhat in a later publication, with the addition of an analysis (see below). As noted therein the mineral also resembles that described by Chudoba as a manganophyllite allied to the phlogopite group, but the fuller description given below reveals important differences.