REPORT ON THE INCLUSIONS OE THE 



YOLCANIC ROCKS OF THE ROSS 



ARCHIPELAGO 



(With Three Plates and Three Figures in the Text) 

 BY 



J. ALLAN THOMSON, M.A., D.Sc, F.G.S. 



Apart from the information that inclusions may give as to the sequence in age of 

 volcanic rocks and the constitution of the underlying terrane, their petrological 

 study may throw important light on pedogenesis. Lacroix, who has made a 

 speciality of these studies for many years, declared in 1893, after the examination 

 of over three thousand specimens : " Cette richesse en materiaux nouveau me permet 

 de tirer quelques conclusions generales de mes observations, bien que j'aie juge necessaire 

 d'etre fort prudent sur un sujet semblable, pour lequel les faits d' observation ne seront 

 jamais trop nombreux."* 



The inclusions of a province petrographically so interesting as that of Ross Island 

 thus possess more than a local interest, and the geologists of the Shackleton Antarctic 

 Expedition are to be congratulated on the extent of their collections. 



Although there is a general agreement as to the nomenclature of certain groups 

 of inclusions, their classification as a whole is still a subject of disagreement among 

 penologists. The various groups will therefore be first described and their origin dis- 

 cussed before the attempt is made to show their place in a general classification. 



Several types of inclusions have previously been collected by Ferrar and the officers 

 of the Scott Expedition, and described by Prior, f Of these all but two were found 

 in the basalts and limburgites near Winter Quarters, and were of the nature of olivine 

 and gabbroid nodules. The two exceptions were enclosed in trachytes, the one being 

 a felspathic nodule or sanidinite, and the other consisting mainly of hornblende. All 

 the types described by Prior are included in the present collection with the exception 

 of the sanidinite from the trachyte of Cape Crozier. 



I. OLIVINE, PYROXENE, AND GABBROID NODULES IN THE BASIC ROCKS 



A large number of olivine, pyroxene, and gabbroid nodules were collected from the 

 basalts and limburgites of Hut Point. In some few cases the specimens were enclosed 

 in the volcanic rock, but more often they were loose, so that the exact nature of the 

 host could not be determined. Owing to the difficulty of preparing thin sections of 

 these loosely compacted rocks, but few slides were made, and the mineralogical com- 

 position was ascertained by crushing small fragments under oil on a glass slip and 

 examining the powder under the microscope. A more permanent preparation may be 



* Lacroix, A., Les Enclaves des Roches volcaniques, Macon, 1893, p. 13. 



t Ferrar, H. T., and Prior, G. T., National Antarctic Expedition, 1901—1; Natural History 

 vol. i; Geology, Brit. Mils. Rep., London, 1907, pp. 11, 12, 14, 106-9, and 114. 



n 131 u 1 



