151 



near Durness, iu Sutherlandshire, is a mass of rock composed of a pinkish 

 saussuritic looking substance and hornblende. On examining a fractured 

 surface of the supposed saussurite, glistening cleavage surfaces may be detected 

 here and there. Under the microscope it is seen to consist of colourless 

 prismatic microlites and a water-clear groundmass which splits up under 

 crossed nicols into large, irregular, untwinned grains. The microlites give 

 straight extinction, and are traversed by cross fractures at irregular intervals. 

 They possess strong refraction and weak double refraction ; or, in other words, 

 their relief is strongly marked, but they give under crossed nicols only the 

 neutral tints of the first order. In all these characters they agree with 

 zoisite, to which mineral we may accordingly refer them. The colourless 

 groundmass possesses the refraction and double refraction of felspar. As 

 already mentioned, distinct cleavage surfaces may occasionally be seen in the 

 saussuritic aggregate. A fragment broken off from one of these and tested 

 by SZABO'S method gave the flame colouration and fusibility characteristic of 

 albite. We may therefore conclude that we have here a saussurite similar to 

 that described by CATHREIX ; that is a mixture of zoisite and albite, with a 

 variable quantity of hornblende occurring as an accessory constituent. 



The rocks of Sangomore Bay have been profoundly affected by regional 

 (pressure) metamorphism, and it seems probable, therefore, that the zoisite 

 and albite have been developed, in connection with this metamorphism, from 

 the basic felspar of a gabbro. 



The Scourie dykes (1) furnish us with interesting illustrations of the 

 modification to which the plagioclase of a basic eruptive rock is subjected 

 under the influence of powerful mechanical stresses. The original felspars 

 give lath-shaped sections. If a series of microscopic slides illustrating the 

 passage of the massive plagioclose-augite rock into a crystalline schist be 

 examined under the microscope, it will be seen that the change is accompanied 

 by a molecular re-arrangement of the felspar. 



This is proved not only by the fact that the original form and internal 

 structure (twinning) of the felspar is lost, but also by the occurrence of 

 secondary hornblende as inclusions in the felspathic aggregates. The manner 

 in which the molecular re- arrangement of the felspar takes place appears to 

 vary under varying circumstances. Sometimes cloudy crypto- or micro- 

 crystalline aggregates are formed ; at other times the mosaic-like aggregate 

 of colourless grains is produced ; while at others the secondary felspar occurs 

 in the form of large irregular grains. Two features are especially striking 

 in the secondary felspar the comparative absence of twinning and its 

 wonderful clearness. Where it occurs in detached grains, or even in 

 aggregates, it is often quite impossible to distinguish it from quartz without 

 the use of convergent light. An important question arises as to the nature of 

 the secondary felspar. At one point, where the more southerly dyke has been 

 greatly affected by the deforming forces, a vein of nearly pure felspar 

 substance occurs. The individual grains are large, irregular in form, and 

 devoid of twinning. They have a specific gravity of 2 '65, and a composition 



(1) TEALL. On the Metamorphosis of Dolerite into Hornblende-Schist. Q.J.G.S., vol. XLL, 

 1885, p. 133. 



