Geological Society of London. 333 



bedding and banding of the gabbro-masses of the Inner Hebrides, 

 the authors describe the rocks which form the rugged ridge of 

 Druim-an-Eidhne, near the head of Glen Sligachan. This ridge 

 is made up of parallel beds, sheets, or sills disposed in a general 

 N.N.W. direction with a prevalent easterly dip. Four distinct types 

 of gabbro occur: — (1) dark, fine-grained, granulitic gabbros ; (2) 

 well-banded gabbros ; (3) coarse-grained massive gabbros ; and (4) 

 pale veins of a highly felspathic gabbro. The relative ages of the 

 banded and granulitic gabbros have not been definitely settled ; but 

 the coarse massive gabbros are certainly intrusive in the banded 

 series, and the pale veins cut all the other varieties. 



The paper deals mainly with the banded gabbros. They occur 

 in successive sheets or sills which vary from a few feet to many 

 yards in thickness, and consist of parallel layers of lighter and 

 darker material which correspond in direction with the trend of the 

 sheets, and are usually inclined to the east or south-east at angles 

 rang-ing: from 20° to 30^. In some cases the bands can be seen to 

 have been puckered or folded. 



The minerals entering into the composition of the banded, as also 

 of the other varieties, are labradorite, pyroxene, olivine, and titani- 

 ferous magnetite. The banding is due to a variation in the relative 

 proportions of the difierent constituents, and especially in the amount 

 of magnetite. Some narrow bands and lenticles are composed 

 entirely of pyroxene and magnetite. The variations in chemical 

 composition are illustrated by three analyses by Mr. Player. The 

 microscopic characters of the rocks are described, and it is shown 

 that the minerals of the banded gabbros have not been crushed or 

 broken since they were formed. 



The authors conclude that the banding is the result of the intru- 

 sion of a heterogeneous magma, and that similar banding in certain 

 portions of the Lewisian gneiss may have been produced in the 

 same way. 



2. " On the Microscopical Structure of the Derbyshire Car- 

 boniferous Dolerites and Tulfs." By H. H. Arnold-Bemrose. Esq., 

 M.A., F.G.S. 



The paper deals with the petrography of the Toadstones or 

 igneous rocks of Derbyshire. Brief reference is made to the work 

 of previous petrographers, the age of the rocks, and the question as 

 to the number of beds. The outcrops mapped by the Geological 

 Survey, and several additional ones, have been examined, and the 

 results given in a table for the purpose of the paper and for future 

 reference. 



The Toadstone is divided into massive rocks or lavas, and frag- 

 mental rocks or tuffs. The former consist of olivine-dolerite, either 

 with granular or with ophitic augite, and olivine-basalt. The rock 

 is often very fresh, but in some places is altered to a diabase. The 

 principal constituent minerals are described. A pseudomorph of 

 olivine, optically like biotite and somewhat like Iddingsite but 

 differing from it chemically, is fully described. 



The latter portion of the paper deals with the tuffs, which are 



