Prof. K. Busz — Corundum on Dartmoor. 493 



interest. Although I have not finished the examination of these, yet 

 I have made some remarkable observations, which I should like 

 to describe to the readers of this Magazine. 



It is well known that the Dartmoor Granite is the largest of 

 the numerous granite exposures of Devon and Cornwall. As to 

 its origin and age, there have been brought forward very many 

 different views and theories. Whatever these may be, it is, 

 I suppose, generally understood that this granite-mass is of post- 

 Carboniferous age. The Devonian, as well as the Carboniferous 

 strata and their interbedded contemporaneous igneous rocks, have 

 undergone great alteration at their zone of contact with the granite. 



The place I visited, where these alterations are well exposed, is 

 near South Brent, on the south end of the Dartmoor Granite, where 

 a little stream, the Avon Biver, cuts right through the contact-zone. 



The rocks affected by the contact-metamorphism are of different 

 characters. I found Devonian clay-slate altered into chiastolite- 

 slate and spotted mica-slate, while small seams of interbedded 

 limestone have given rise to the formation of garnet and other 

 minerals. 



I may mention here that I found close to South Brent on the 

 right side of the Avon Biver one of these limestone-layers, of about 

 two yards' width, almost entirely changed into garnet. This garnet, 

 grossularia (calcium-iron-garnet), is of a light-brown colour, and 

 either shows the very well-developed form of the dodecahedron or 

 occurs in granular masses with no distinct crystallographic form ; 

 it shows a very strong, anomalous, double refraction, and also very 

 distinctly the partitioning into sectors described by Professor Klein. 

 The crystals are sometimes imbedded in a saccaroid-grained white 

 mineral, which chemical analysis proved to be the lime-felspar 

 anorthite. Besides these, malacolite and axinite occur abundantly 

 in this locality. 



It may perhaps be of interest to mention that I have found 

 Cassiterite to be present in some of the andalusite-bearing schists. 

 It occurs not only imbedded in the long-shaped crystals of andalusite 

 but also in the groundmass of the rock, and exhibits well-defined 

 crystalline forms, which very often show the well-known twinning. 



By the aid of hydrofluoric and hydrochloric acid I succeeded in 

 isolating these crystals for chemical analysis. 



All these alterations of the schists surrounding the granite, which 

 we may call exomorphous contact-metamorphism, are of course very 

 common, and can be observed more or less intensely developed in 

 every granite-mass. Not so common, but, on the contrary, rather 

 rarely observed, is endomorphous contact-metamorphism, and it is 

 the latter to which I desire to draw the attention of your readers. 



In the above-mentioned locality I found in close contact with 

 clay-slate a felsitic porphyiy, which, in a light grey-coloured 

 groundmass, contained a few small but rather well-defined crystals 

 of plagioclase. One specimen which I was able to collect consists, 

 one half of this felsitic porphyry, the other half of clay-slate 

 altered into black hornfels. The latter is very much broken, 



