328 J. ARTHUR PHILLIPS ON THE ROCKS OP 



I. II. 



Water* j hygrom^rio -70 -68 



| combined 3-10 2-91 



Silica 49-90 49-93 



Phosphoric anhydride trace trace 



Alumina 21-09 21-23 



Ferrous oxide 11-93 11-86 



JFerric „ -72 -88 



Manganous oxide , trace trace 



Lime -68 -75 



Magnesia 6-86 6-84 



Potassa , . -63 -57 



Soda.. ..'. 4-34 4-32 



99-95 99-97 



An examination under the microscope of thin sections of this 

 deposit shows it to be an ash-bed, composed of fragments of vesicular 

 lapilli having a very similar composition to that of the lava from 

 Trelill quarry. In this rock, however, the vesicles, which are 

 smaller but more numerous than in the other, contain a larger pro- 

 portion of chlorite. 



It will be remarked that in chemical composition the two rocks 

 do not materially differ, except that the latter contains a larger 

 amount of magnesia. In both cases, however, there can be no doubt 

 that a portion of the material now filling the vacuities has been de- 

 rived from a partial decomposition of the original rock. 



Other " Greenstones" 



In addition to the foregoing, a great many very different rocks 

 receive the name of "■ greenstones." Of these a large proportion are 

 only more or less altered hornblendic or chloritic slates, while others 

 are of igneous origin. They are, however, so numerous, and so dif- 

 ferent in character, that a more detailed enumeration of them would 

 not be consistent with the limits of the present paper. I shall now, 

 therefore, describe only two well-known greenstones, leaving to a 

 future period the examination of some other varieties. 



Greenstone from Botallack. — This rock extends, in the form of a 

 narrow band, along the sea-cliff from Porthleden Cove to a little 

 north of the Levant Mine. It is sometimes schistose, and at others 

 apparently amorphous ; the change from the one condition to the 

 other taking place by imperceptible gradations. Its colour is a dull 

 sage-green ; and a recent fracture exhibits some indistinct crystal- 

 line facets. 



An analysis, in duplicate, of an unweathered specimen of the 

 amorphous variety of this rock afforded the following results (sp. 

 gr.=2-96):— 



* Estimated as in preceding analysis. 



