264 



strongly dichroic This variety of the rock is therefore a hornblende- 

 augite-biotite-diorite. 



Prof. ZIRKEL mentions the fact that the rock contains rounded concre- 

 tionary masses, somewhat finer in grain and richer in hornblende than the 

 main mass. The occurrence of such concretionary ( ?) masses is a common 

 feature both in the diorites and the granites. 



Dioritic rocks occur as intrusive sheets in the limestones and quartzites 

 of the Assynt district. The hornblendes of these rocks are remarkable for 

 their zonal structure and for the perfection of their forms. They appear to 

 be always idiomorphic with respect to the felspar. Crystals of hornblende 

 detached from the rock show the forms (110), (010), (111) and (001). In thin 

 sections the form (100) may also be occasionally observed. The pleochroism 

 is well marked: a, pale green to yellowish green; /Sand 7, deep green 

 with only slight differences. Twinning is very common. Sometimes a 

 crystal may be seen to be composed of three or four lamellae The altera- 

 tion of the hornblende is accompanied by the formation of chlorite, epidote 

 and opacitc. In the diorites proper, as distinguished from the hornblende- 

 porphyrites, the felspar occurs in irregular grains and appears to belong- 

 only to one period of consolidation. The felspar-aggregate plays the role 

 of ground-mass, and the well-formed hornblende crystals lie in it without 

 any regard to the orientation of the individuals of which it is composed. 

 The coarseness of the felspar- aggregate varies within wide limits, sometimes 

 it is so fine as to merit the term micro-crystalline ; at other times it is so 

 coarse as to remind one of the structure of a medium-grained granite. 

 Pyroxene is not constantly present in these rocks. It is pale green when 

 examined on a fractured surface of the rock, but colourless in thin section. 

 It occurs in somewhat imperfect crystals and also as grains and granular 

 aggregates. The forms of the crystals, so far as they are developed, appear 

 to be those of the common rock-forming augites. The maximum extinction 

 in the prismatic zone is about 4*0. The only cleavages recognized are those 

 of the prism. The mineral has been especially noticed in rocks which are 

 intrusive in the dolomitic limestone ; but it varies very much in quantity 

 even in these, sometimes being absent altogether, and at other times 

 occurring almost as abundantly as the hornblende. The other minerals are 

 magnetite, apatite, calcite, epidote and quartz. The quartz appears to be 

 in general a secondary product. The calcite is present in large crystalline 

 plates which, in some cases, fill up the interspaces between comparatively 

 unaltered felspar. 



The above minerals occur in very different proportions in the different 

 classes of hornblende-bearing rocks from the neighbourhood of Inchna- 

 dampf. In some the hornblende is very abundant. These have as a rule 

 a granular texture and may be termed diorites. They tend to become 

 porphyritic in places by the conspicuous development of hornblende 

 crystals. The following analyses represent the composition of two varieties 

 of diorite. The first is free from pyroxene or contains it only in very 

 small quantity ; the second is rich ip pyroxene. A comparison of the two 



