184 DREDGINGS OF THE MARINE BIOLOGICAL ASSOCIATION: 



Four Dioritb. Nos. 1, 16, 22, 62. 



H. 1. From Salcombe fishing grounds. Weight 9^ cwt. 



Macroscopically this rock shows a pale yellowish-white felspar and a very 

 dark green hornblende, which appear to be rather closely set in a dull yellow- 

 grey rather compact matrix. The slide shows felspar crystals, which have a 

 tolerably regular outline, but are, as a rule, much decomposed, the mineral 

 being often converted into an aggregate of earthly granules. In some of the 

 crystals the polysynthetic twinning of plagioclase is still visible. The horn- 

 blende is a rich green colour, with fairly strong dichroism, and tolerably per- 

 fect crystal outlines. There are several grains of quartz, and a few clusters of 

 small flakes of brownish mica, and a little magnetite. — T, G. B. 



H. 16. Trawled 17 miles S. of Start Point. Measures 2' 6" x 1' 10" x 1' 2". 



A sap-green coloured rock, in which the large actinolite crystals chiefly 

 catch the eye ; it is coarsely crystalline. The microscope shows the long ac- 

 tinolite crystals, green in colour, and at the borders often connected with 

 diverging bundles and needles of pale green crystals, also actinolite, which 

 penetrate the felspars. 



The plagioclase still preserves its twinning for the most part, but much of it 

 is attacked by decomposition, and it is everywhere permeated by long actino- 

 litic fibres and particles. Apatite is present. Secondary quartz has been 

 deposited in little veins and interstices, — E. B. T. 



H. 22. Trawled 20 miles S. of Eddystone. Weight about 5 cwt. 



Quartz is not so abundant as in granite, while the microscopic examination 

 shows that the prevailing felspar is triclinic. Hornblende is abundant, and 

 also dark mica; the latter occurs not so much as scattered crystals as in 

 groups of diverging or matted prisms. Apatite is abundant ; magnetite grains 

 occur also mixed with the mica. — E. B. T. 



H. 62. Erratic on shore near Gorah Run, E.S.E. of Prawle. 



A moderately coarse granitic rock, in which black hornblende is conspicuous, 

 set in white felspar and grey quartz. Lustrous flakes of dark mica are also 

 seen. 



Green hornblende and brown biotite are both well represented. The 

 former often shows faces of the prism-zone, but never builds very perfect 

 crystals. One section, three-eighths inch in diameter, is studded with little 

 grains and rounded crystals of felspar, and some of quartz. The smaller cry- 

 stals of hornblende are sometimes tAvinned on the usual law. Much of the 

 biotite, which tends to build stout flakes, is bleached and partially de- 

 composed. The felspar tends to form rectangular crystals, and is chiefly, 

 if not wholly, oligoclase with rather close albite-lamellation. The crystals 

 are often cloudy from alteration, especially in the interior. The clear 

 quartz often shows strain-shadows. The only other original mineral is a 

 little apatite. 



Quartz is more abundant than in most ordinary quartz-diorites, — A. H. 



