540 Geological Society. 



in a mass to some extent heterogeneous. This hypothesis they 

 consider may be applied to all gneisses or schists which exhibit 

 similar structures — that is, to a considerable number (but by no 

 means all) of the Archsean rocks. 



The second part of the paper consists of notes on some of the 

 dykes and obviously intrusive igneous rocks of the Island. Among 

 these are four (new) dykes of " mica-trap," one of which exhibits 

 a very remarkable "pisolitic" structure. The variety of picrite 

 described by Prof. Bonney in 1889 (from a boulder in Port du 

 Moulin) has also been discovered in situ. 



2. " On the Plutonic Rocks of Garabal Hill and Meall Breac." 

 By J. B. Dakyns, Esq., M.A., and J. J. H. Teall, Esq., M.A., E.B.S., 

 P.G.S. 



The plutonic rocks described occur in a complex forming a belt 

 of high ground S.W. of Inverarnan. They vary considerably 

 in composition, and though gradual passages are sometimes found 

 between more or less acid rocks, at other times the junction is 

 sharp. The more acid are always found to cut through the less 

 acid when the two rocks are found in juxtaposition, and fragments 

 occurring iu a rock are less acid than the rock itself. Though thus 

 shown to be of different ages, they must evidently be referred to one 

 geological period. The first rocks to be formed were peridotites ; 

 then followed diorite, tonalite, granite, and eurite in order of 

 increasing acidity. 



The specific gravities, colours, and textures of the rocks are con- 

 sidered, and a detailed account of the constituent minerals given. 

 The essential minerals are arranged in the following order, based on 

 their general distribution in the different types of rock : — Olivine, 

 pyroxene, hornblende, biotite, plagioclase, orthcclase and quartz, 

 microcline. The following is the order in which the principal con- 

 stituents commenced to form in the rocks : — Iron-ores, olivine, 

 pyroxene, hornblende, biotite, plagioclase, orthoclase, microcline, 

 and quartz. The chemical composition of the rocks is discussed, 

 data being furnished by a series of analyses made by Mr. J. H. 

 Player, and a diagrammatic representation of the molecular relations 

 of the different bases and silica is given. The relations between 

 mineralogical composition, chemical composition, and geological age 

 are then considered : and the following conclusions are reached : — 



(1) That the various rocks have resulted from the differentia- 



tion of an originally homogeneous magma. 



(2) That the chronological sequence from peridotite to eurite 



is connected with the order of formation of minerals in 

 igneous magmas. 



3. " North Italian Bryozoa. — Part II. Cyclostomata." By Arthur 

 Wm. Waters, Esq., F.G.S. 



