SECTIONAL TRANSACTIONS.— C. 419 



study of the dolerite sill of Fair Head, Co. Antrim. In this sill olivine has 

 the following average composition : (i) from the glomeroporphyritic 

 aggregates (allivalite) representing an early phase of crystallisation — 

 24 % fayalite ; (2) from the dolerite — 43 % fayalite ; (3) from the dolerite- 

 pegmatite schlieren, a late phase— 71 % fayalite. 



There seems also to be some relation between the variation in the 

 composition of olivine and that in the composition of the rock in which it 

 occurs, an increase in the iron content of the olivine being usually accom- 

 panied by a similar increase in the alkalies and silica content of the rock. 



The parallelism between the zonality of olivine crystals, the variability 

 of the composition of olivine in the successive magmatic phases and the 

 relation between the composition of the olivine and the magma, has a 

 definite bearing on the question of petrogenesis. The recorded observa- 

 tions — which are in perfect agreement with the recently published study on 

 the forsterite-fayalite binary system by Bowen and Schairer — show the 

 possible control exercised by olivine on magma and the relation between 

 the composition of the olivine and that of the magma. 



Dr. F. Walker. — The dijferentiation of the Palisade diabase sill, New 

 Jersey (11,10). 



A detailed quantitative examination of the best exposed sections through 

 the famous Palisade Diabase Sill leads to the following conclusions : 



(i) It is doubtful whether the ' olivine layer ' in the lower part accum- 

 lated by gravitational settling of olivine. 



(ii) There is a definite gradational concentration of pyroxene above the 

 ' olivine layer,' indicating sinking of pyroxene. 



(iii) Assimilation of arkose on a large scale is improbable. 



(iv) The proportion of free silica in the sill as a whole is very small, but 

 there is a marked concentration just below the upper chilled phase. 



(v) The magmatic history of the sill ended with pronounced hydro- 

 thermal activity. 



Dr. A. Wade and Dr. R. T. Prider. — The geology and petrology of the 

 leucite rocks of the Kimberley district, Western Australia (11.45). 



Nineteen occurrences of post-Permian volcanic rocks have been found 

 in the West Kimberley area. Plugs, partly eroded craters and fissure 

 intrusions have been recognised. The structure and distribution of these 

 occurrences indicate that the intrusions have ascended along fault planes 

 which are connected with the structure of the underlying pre-Cambrian 

 rocks. 



The rocks are made up of varying proportions of leucite, phlogopite, 

 diopside, simpsonite (a new K-Mg. amphibole related to katophorite), 

 wadeite (a new K-Zr silicate), rutile, chlorite and indeterminable ground- 

 mass. Four new rock types (fitzroyite, cedricite, mamilite and wolgidite) 

 are described. 



Although leucite is developed to the complete exclusion of sanidine, the 

 rocks contain more than sufficient silica to have formed orthoclase instead 

 of leucite. The magma, from which these rocks crystallised, was of 

 peculiar character — its main features being high potash dominant over 

 alumina, high magnesia and titania and very low soda content. The minor 

 constituents are comparatively abundant. 



This magma was probably derived from a potassic mica-peridotite magma 



