" GREENSTONES " OF WESTERN CORNWALL. 



165 



this rock is almost entirely composed of closely felted crystals, 

 among which is disseminated a little flocculent viridite, or more 

 rarely magnetite; not unfrequently the hornblendic mineral is 

 thickly distributed through a colourless slightly opaque base, 

 which forms but a small proportion of the whole. Patches of 

 quartz may also occasionally be observed ; and into these minute 

 green crystals project from the surrounding mass like the foliage of 

 a tree into the open spaces between its branches. In addition to 

 green hornblende, and perhaps a little chlorite, these rocks fre- 

 quently contain brown hornblende, together with colourless crystals 

 of a mineral which is probably tremolite. Magnetite is likewise 

 sometimes present ; but instead of being in a crystalline form, as in 

 the case of the rocks before described, it is generally disseminated 

 either as a sooty powder or in rounded aggregations. 



Twenty-six different sections of these rocks were cut in various 

 directions, and examined under the microscope with the foregoing 

 general results, while four specimens were subjected to chemical 

 analysis ; in each case the mean of two separate estimations is given 

 in the following table ; and a description of the appearance of thin 

 sections, seen under the microscope, is appended. 



Table slwwing the Composition of Four Varieties of Crystalline 

 Slaty Rode. 



txT , f hvsrrometric 



I. 



II.* 



III. 



IV. 



•54 



•77 



3744 



•27 



1G'32 



7-33 



•23 



14-75 



trace 



12-83 



6<06 



1-50 



1-85 



1-37 

 1-56 



35-58 

 distinct trace 

 21-14 

 14-74 

 traces 

 10-32 



trace 



9-20 



2-78 

 •95 



2-11 



•63 



•87 

 39-20 



•74 

 16-22 



4-82 



15-88 



10-70 

 6-56 

 2-69 

 1-45 



•68 



106 



36-57 



•68 



15-34 



7-57 



14-84 



trace 



12-58 



5-98 



1-44 



2-99 



Water { combined 



Silica 



Phosphoric anhydride 



Ferric oxide 



Ferrous oxide 







Magnesia 





Soda. . . , , 







99-89 



99-75 1 99-66 



99-73 





3-29 



315 



3-26 



3-28 









I. Paul Hill, Paul. — This rock is hard, dark in colour, very fine 

 in grain, and possesses a distinctly slaty structure. Thin sections, 

 examined under the microscope, are found to be composed chiefly of 

 minute thickly matted green crystals, through which are disse- 

 minated a little flocculent viridite, and occasionally some dust-like 

 magnetite. It encloses small irregular patches of milky or trans- 

 parent quartz, traversed by minute hornblendic crystals. 



* This analysis was published in the Quart. Journ. Geol. Soc. vol. xxvi- 

 p. 329, but is introduced here for the sake of comparison. 



