•334 Reports and Proceedings — Geological Society of London. 



The relative age of some of the peculiar types of intrusive 

 rocks is indicated in the paper in those cases in which it can be 

 determined. That those rocks which are later in date than the 

 folding of the Ordovician are older than the Upper Old Eed Sand- 

 stone is shown (1) by the unconformity of the Upper Old Eed 

 Sandstone ; (2) by the fact that the latter rock does not contain any 

 interbedded igneous rocks; and (3) by the absence of felsitic or 

 other intrusive rocks from the Old Red Sandstone of the district. 



The second part of the paper is devoted to petrological notes on 

 the different rock-types. The felsites are classified by means of 

 their groundmass into microcry stall ine, cryptocrystalline, and micro- 

 poikilitic and other types ; potash-felsites, potash-soda-felsites, and 

 keratophyres, all appear to be present ; some of these rocks are 

 linked to the trachytes and bostonites. The diabases and dolerites 

 are sometimes ophitic, but more usually allotriomorphic in texture. 

 The trachytes and andesites are of various types, and some are 

 probably keratophyres. Quartz- and felspar - porphyries, augite - 

 porphyrites, and several miscellaneous types are also described. 



2. " On a new Type of Rock from Kentallen and elsewhere, and 

 its Relations to other Igneous Rocks in Argyllshire." By J. B. Hill, 

 Esq., R.N., and H. Kynaston, Esq., B.A., F.G.S. (Communicated 

 by permission of the Director-General of H.M. Geological Survey.) 



A rock originally described by Mr. Teall from Kentallen is used 

 by the authors as a type round which they group a peculiar series 

 of basic rocks discovered in several localities. The rocks consist 

 essentially of olivine and augite, with smaller amounts of orthoclase, 

 plagioclase, and biotite, while apatite and magnetite are accessory. 

 The peculiar feature of the rocks is the association of alkali-felspar 

 with olivine and augite, and the group is related to the shonkinite 

 of Montana and the olivine-monzonite of Scandinavia. The occur- 

 rence of the rocks is connected with four neighbouring but distinct 

 areas of intrusion, each characterized by granites and diorites, and 

 by dykes and sills of lamprophyres, porphyrites, etc. In these areas 

 the new rock is the most basic type, and it occurs in the marginal 

 portions of the areas. Close relationships exist between the different 

 intrusive rocks in each area, so that it may be concluded that these 

 constitute a ' rock-series ' ranging from granite through augite- 

 diorite towards the olivine-bearing rocks, in the plutonic phase, and 

 from orthoclase-porphyry and porphyrite to augite-lamprophyre, in 

 the dyke-and-sill phase. The whole assemblage appears to have 

 been derived by a process of differentiation from one parent magma ; 

 and the order of intrusion has been, in the main, one of increasing 

 acidity. There is further a ' facies-suit ' in each intrusion, showing 

 progressive increase in basicity from centre to magma, due to con- 

 centration of the more basic oxides in the cooler portions of the 

 magma, which was originally of intermediate composition ; from 

 this 'complementary rocks' wei-e produced. It is extremely probable 

 that the underlying magmas of the four intrusive areas resembled 

 one another more or less closely in composition. 



