444 Geological Society. 



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 penological notes on 

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

 their groundmass into microcrystalline, 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 II. Kynaston, Esq., B.A., F.G.S. 



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 augitewith 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, porphy rites, 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 pa/rent 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 ' were produced. It is extremely probable 

 that the underlying magmas of the four intrusive areas resembled one 

 another more or less closely in composition. 



