352 J. J. Harris Teall — Hornblende-Bearing Bocks. 



blende. There are, however, some important points of difference. 

 The original hornblende of the Warwickshire rocks is brown, and 

 where it occurs abundantly is in the form of large ophitic plates con- 

 taining inclusions of lath-shaped felspar. In the Assynt rocks the 

 hornblende is green, and appears in all cases to have preceded the 

 felspar in the order of consolidation. Again, some of the Warwick- 

 shire rocks contain numerous pseudomorphs after olivine, whereas 

 no trace of this mineral has been detected in any of those from 

 Assynt. The ophitic character of the hornblende and the presence 

 of olivine both point to the conclusion that some of the Warwick- 

 shire rocks are more basic in composition than any yet found in 

 Assynt. Many of the Anglesea " diorites " bear the closest relation 

 to the corresponding rocks from Warwickshire. They consist of 

 brown hornblende often passing into green, a colourless pyroxene, 

 plagioclase and magnetite or ilmenite. By the coming-in of olivine 

 and the dying-out of the felspar, these rocks shade into Professor 

 Bonney's hornblende-picrite. Mr. Tawney l has described similar 

 rocks from the Lleyn peninsula, Caernarvonshire, under the name of 

 hornblende- diabase. An interesting little boss of " diorite " occurs 

 on the right-hand side of the footpath leading from Mount Sorrel to 

 Swithland (Leicestershire), and just outside Brazil Wood. 



This boss projects through the soil, and measures only a few yards 

 across. It varies considerably in texture, some parts being com- 

 posed of a medium-grained granular rock, and others containing 

 large hornblende crystals or plates. Prof. Bonney has described 

 the rock of this knoll as follows : " Under the microscope the latter 

 mineral [hornblende] is seen to occur in irregular grains, plate-like 

 in section, pierced and intersected by the plagioclase [ophitic] with 

 a little iron peroxide (? some of it ilmenite). The hornblende with 

 transmitted light is both brown and green in colour, and much of it 

 and of the felspar is still in good condition. Here and there patches 

 of the pale fibrous actinolitic variety occur, associated with a mineral 

 rather like a fibrous variety of serpentine. These last are certainly 

 secondary products, and much resemble a variety common in the 

 altered Cornish gabbros. Most of the larger hornblende is strongly 

 dichroic. Some of the brown variety resembles biotite. One or 

 two pieces indeed appear from their optical properties to be that 

 mineral. There is a little calcite with characteristic cleavage. The 

 plagioclase often shows brilliant colours and characteristic twinning, 

 with occasional indications of a zonal structure." 



My slide of rock from the same locality agrees with Prof. Bonney's 

 description, but contains, in addition to the minerals which he has 

 mentioned, a colourless pyroxene. This brings the rock into the 

 closest relation with some of Mr. Allport's diorites from Warwick- 

 shire, with the Anglesea " diorites," and with Mr. Tawney's horn- 

 blendic-diabases from the Lleyn peninsula. I would suggest that, for 

 convenience of description, we should use a special locality as a 



1 Woodwardian Laboratory Notes, Geol. Mag. Decade II. Vol. VII. pp. 212 and 

 457; also Vol. IX. p. 548. 



