270 A COMPOSITE SILL AT XEWTOX ABBOT. [vol. Ixxviii., 



may be compared with the crystals in fig. 3. Crossed nicols, 

 X 30 diameters. (See p. 266.) 

 Fig. 5. Olirine-dolerite (X 22). Edge of field, Knowles Hill. Olivine- 

 crystal in the centre, orthoclase on the left. The remainder of the 

 slide consists of andesine, augite, and quartz. Ordinary Hght, X 30 

 diameters. (See p. 268.) 

 6. Dolerite (X18). Summit Quarry, Knowles Hill. Augite subophitic 

 to andesine : quartz on the right. The iron-ore is ilmenite. Crossed 

 nicols, X 30 diameters. (See p. 267.) 



Discrssiox. 



Dr. J. S. Flett Tvelcomecl this paper as a contribution to the 

 peti'OgTaphy of the greenstones of Corn^Yall and Devon. Thfr 

 diabases of that province not unfrequently showed a tendency to 

 develop an ' alkaline ' facies, especially in the abundance of 

 felspars rich in soda ; but, in the speaker's experience, many of 

 them had been extensively ' albitized,' either dm*ing crystallization 

 or shortly after consolidation. The abundance of albite was note- 

 worthy also in the pillow-lavas and keratoph}T.'es, which he supposed 

 were the effusive representatives of the same magma. Analcite 

 was very i-are in these rocks, and it seemed possible that albiti- 

 zation occurred in place of analcitization when the post-volcanic 

 solutions emanating from the magma were comparatively siliceous. 



]Mi'. L. Hawkes said that two types of quartz from the dolerite 

 were described, and it was sus'O'ested that the laro^er individuals 

 with corrosion-embayments had been derived from a sandstone. 

 The speaker asked whether these crystals were scattered evenly 

 throuo'hout the rock, and whether thev alwavs occmTcd sins^lv. 

 From, the origin indicated by the Author one would have expected 

 to find iiTegularly distributed fi-agments, some built of more than 

 one individual. The speaker had found big corroded quartz- 

 grains quite commonly in basic rocks in Iceland, where no sand- 

 stones were known, and had come to the conclusion that they had 

 either crystallized from the basic magma, or had been incorporated 

 from an acid one, in some way not yet satisfactorily explained. 



Mr. A. K. Wells said that he had difficulty in agreeing with 

 the use of certain rock-names for some of the specimens exhibited. 

 In particular he would like to ask the Author whether his 

 ' mugearite ' showed any of the structural and textural pecu- 

 liarities characteristic of the type-rock as defined by Dr. A. Harker 

 fi'om Skye. As Harker's 'mugearite" was a well-individualized 

 type, it was perhaps undesirable to apply the term to a rock which 

 appeared to resemble tlie type in one particular only — the compo- 

 sition of the felspar. 



The ArTHOE expressed his thanks for the reception accorded to 

 his paper. In reply to Mr. Hawkes, he stated that most of the 

 quartz Avas primary, a few isolated and coiToded crystals only 

 being considered secondary. In reply to the criticism of nomen- 

 elatm-e by Mr. Wells, he said that ' mugearite ' had been used as a 

 generalized term for oligoclase-dolerite, and 'bostonite" for an acid 

 differentiate of a basic magma. 



