492 C. E. Tilley — Metmnorphison of 



thrust along. It is contemporaneous with the other conglomerates 

 overlying I), limestones round the north-western and western 

 margins of the Midland Limestone massif. The formation of these 

 conglomerates and the erosion of the Caldon channel appear to have 

 been the result of earth-movement in late, or post, D2 times, evidence 

 of which is present in the eastern part of the Midland area, near 

 Youlgreave and Darley Dale. 



The Metamorphism of the Pre-Cambrian Dolomites of 

 Southern Eyre Peninsula, South Australia. 



By C. E. Tilley, B.Sc, A.I.C, Emmanuel College, Cambridge. 



{Concluded from i^- 463.) 

 (PLATE XL) 



The Calc-magnesian Silicate Rocks op the Hutchison Series. 



rpHE rocks arising from the metamorphism of the more impure 

 -■- dolomitic sediments are calc-magnesian silicate types in which 

 all carbon dioxide has been expelled. In some of the rocks to be 

 described it is clear that superposed on the normal metamorphism 

 there has been a metamorphism by addition of material from the 

 neighbouring magma. 



The best exposures of these calc-magnesian silicate rocks are at 

 Sleaford Bay, and they are here closely associated with the dolomites 

 of this area. Fragments of this group have been recognized at 

 Fishery Bay to the south, where they occur as bars in the Flinders 

 gneiss, and they are undoubtedly portions of the Hutchison series 

 engulfed by this intrusion. 



The rock-types include Diopside rock, with or without quartz or 

 l^lagioclase ; Diopside-Microcline rocks with scapoliteorplagioclase 

 as important accessories. Zoisite, titanite, biotite, and graphite 

 are occasionally important constituents. 



(a) Diopside Rock. 



This rock forms a prominent headland on the coast of Sleaford, 

 near Sleaford Mere, and bounding section 14a. The interbedded 

 rocks here strike north with a dip of 80 degrees west, to vertical. On 

 the west the diopside rock is bounded by impure diopsidic beds 

 which pass into mica and mica-graphite schists, and on the east 

 it is bounded by garnet gneisses. The whole diopside bed is 

 intimately veined by quartz strings, which traverse it in sill-like 

 fashion. On the transverse joint faces in veins and in vugs there 

 are present well-develoj)ed jprisms and needles of actinolite. 



Under the microscope the rock is seen to consist almost exclusively 

 of a colourless to pale-green pyroxene, with a high extinction angle. 

 It is non-pleochroic, and the individual grains only occasionally 

 show crystallographic boundaries. The rock as seen in hand- 

 specimens is of a pale-green colour, and may vary considerably 

 in texture from place to place. The grain size is, however, usually 



