231 



RUTLEY make it highly probable that such a substance existed. It is now 

 represented by a greenish-yellow or brownish-yellow substance (palagonite), 

 often crowded with vesicles. Many of the lavas show good fluxion structure 

 in thin section. They also frequently possess schistosity which runs parallel 

 with that of the adjacent sedimentary rocks. 



The tuffs, so far as their composition can now be ascertained, consisted 

 of broken plagioclase-crystals, vesicular lapilli and small fragments of 

 rock similar to the compact lavas. Vesicular and. minutely disintegrated 

 basic glass probably entered largely into their composition. Tufaceous 

 material may be recognized in many of the sedimentary rocks. 



The following analyses by Mr. PHILLIPS will give an idea of the com- 

 position of the lavas and tuffs and at the same time illustrate the very 

 large amount of alteration which has taken place in consequence of the 

 percolation of water charged with carbonic acid : 



I. II. III. 



Si0 2 ... 43-23 ... 40-05 ... 36'74 



AU) 3 ... 21-37 ... 20-46 ... 17'55 



Fe 2 3 ... 1-69 ... 1-83 ... 1-62 



FeO ... 9-53 ... 12-66 ... 8'23 



MnO ... tr. ... tr. ... tr. 



CaO ... 6-66 ... 6-62 ... 12-08 



MgO ... 3-57 ... 4-28 .. 6'04 



Na 2 ... 5-63 ... 4-82 ... 3'08 



K 2 ... tr. ... tr. ... -74 



C0 2 ... 2-61 ... 3-67 ... 9-10 



P 2 5 ... -07 ... -63 ... -35 



FeS 2 ... -33 ... tr. 



' H 2 ... 4-49 ... 4-86 ... 4-31 



100-08 99-88 99-84 



Sp. Gr. ... 2-82 2-83 2-82 



I. Amygdaloidal lava from Pentire Point. 

 II. Schistose amygdaloidal lava from Port Isaac. 

 III. Rock from Bokelly, in the Parish of St. Kew, resembling an indurated ash. 



The more distinctly crystalline rocks, most of which were probably 

 intrusive, have been much less affected by surface agencies than the un- 

 doubted lavas and tuffs. They have, however, been profoundly modified 

 by contact and dynamic metamorphism. As a result we frequently find 

 one and the same rock-mass exhibiting considerable variations of structure 

 and composition, In dealing with these rocks it is of the utmost import- 

 ance that we should distinguish, if possible, between those which contain 

 original and those which contain secondary hornblende. In this work we 

 regard the deep brown " basaltic " hornblende as original. Now this occurs 

 in a few localities and especially in the neighbourhood of Saltash. It is 

 found intergrown with augite and also in independent crystals which 

 show the clino-pinacoid (010) as well as the prismatic faces (Oil), Every 

 other kind of hornblende in the rocks now under consideration we regard 

 as secondary. This secondary hornblende is usually fibrous (uralitic) or 



