182 DREDGINGS OF THE MARINE BIOLOGICAL ASSOCIATION: 



The rock consists mainly of felspar and quartz, with subordinate liiotite, 

 etc. The felspar is chiefly, if not wholly, of triclinic varieties. jNluch of it 

 is microcline ; there is also some oligoclase with carlsbad, and albite-twinning. 

 Most is clear, but there are cloudy patches in places, which seem due to the 

 development of white mica in minute scales. Quartz occurs in large and 

 small grains, usually composite ; strain-shadows are common. The biotite has 

 a marked parallel orientation throughout the slice. It is a deep brown, 

 intensely pleochroic mica, becoming green only by alteration. The other 

 elements of the rock are rare magnetite and green hornblende, with some 

 epidote and other secondary minerals. — A. H. 



Two HoRNBLBNDic Gneiss. Nos. 33, 44. 



H. 33. Trawled about 12 miles S.E. of Start Point. 



Quartz abundant in irregular aggregated granules, felsjiar in occasional 

 grains, with very irregular outline ; orthoclase (probably) and plagioclase. 

 Green hornblende, a strongly dichroic variety, in streak-like aggregates of long, 

 slightly fibrous prisms, magnetite, a few films of brown mica, a little apatite, 

 possibly zircon. — T. G. B. 



H. 44. Trawled block, lying on Brixham Quay. 



A medium-grained felspar-hornblende rock with well-marked banding, the 

 white felspathic and black hornblendic bands being commonly from one- 

 twentieth to one-eighth inch in Avidth. There is no evident fissile structure, 

 and the rock is perhaps to be styled a hornblende-gneiss rather than a horn- 

 blende-schist. 



Felspar, the dominant mineral, is exclusively plagioclase, apparently a basic 

 labradorite. Inconstant twinning is often seen to be clearly connected with 

 a slight bending of the crystal, and must in great part be secondary and the 

 consequence of strain. ]\Iost of the felsj)ar is perfectly clear, but there are 

 also cloudy opaque patches, white by reflected light. The abundant green 

 pleochroic hornblende is in ragged or irregularly bounded crystals. Asso- 

 ciated with it is a clear colourless augite. This is often embedded in 

 the hornblende, but there is no clear indication of the latter mineral 

 having originated at the expense of augite. JSTo iron-ore appears in the 

 slice. — A. H. 

 Three Hornblendic Granitoid Gneiss. Nos. 31, 32, 41, 



H. 31. TraAvled 18 miles S.S.W. of Start Point. Weight about 3 cwt. 



Quartz, felspar, brown mica, a little hornblende, and a little green chloritic 

 mineral, perhaps an alteration product after some of the mica, some apatite. 

 The quartz has rather numerous minute cavities, some empty, some with small 

 moving bubbles. The felspar (which is a little decomposed) is partly ortho- 

 clase, but there is a good deal of albite or oligoclase. — T. G. B. 



H. 32. Trawled 12 miles S.S.E. of Eddystone. Weight about 7 cwt. 

 Minerals as in 31, but in rather different proportions. For instance, there 

 is more hornblende. The state of preservation is not so good. — T. G. B. 



