204 THE GEOLOGICAL AGE OF CENTRAL AND WEST CORNWALL. 



It does not seem at all unlikely that these widely extended 

 intrusions of eruptive rocks resulted from earth-movements, 

 which were preceded and accompanied by the outflow of much 

 highly mineralized water, and which were the direct precursors 

 of the final granitic upheavals. It is true that the basic 

 eruptive rocks of the northern and eastern areas differ some- 

 what in mineralogical composition from the mica-traps — but 

 this difference is probably due to a difference in the amount of 

 pressure under which they became consolidated, or to different 

 rates of cooling. The eastern rocks, as is well-known, are 

 frequently vesicular and amygdaloidal, while those of "West 

 Cornwall scarcely ever exhibit any trace of such structures. 

 But the chemical composition of the rocks of the two areas is 

 so similar that we may well believe them to have come from the 

 same deep-seated source, as will appear from the following 

 table, in which I is the mean of the four analyses given in 

 Table 1, while m is an analysis of the Wearde Rock, n that of 

 Hendra Chapel, and o that of the lava ("dunstone" locally) of 

 Pentire Point — these latter three being the analyses of Mr. 

 J. A. Phillips,* who calls m and n altered dolerites. 



5-39 



I. 

 Moisture . . . . 0-40 

 Combined Water 4-99 



Silica 48-13 



Alumina . . .... 19-68 



Oxides of Iron . . .. 5-39 



Lime 5-05 



Magnesia 5*47 



Alkalies 4-99 



Carbonic Acid . . . . 3-15 



0-32 

 2-02 



Phosphoric Acid 



traces 



97-25 



2-34 



46-42 

 20-23 

 10-32 

 5-99 

 3-82 

 6-95 

 2-32 

 0-98 



99-37 



44-69 

 17-58 

 11-62 

 1054 

 9-81 

 2-87 



99-99 



4-49 



43-23 

 21-37 

 11-22 

 6-66 

 3-57 

 5-63 

 2-61 



98-78 



The principal veins of mica-trap on the one hand, and of 

 the lavas and dunstones referred to on the other, appear to 

 diverge from a point in the sea a little to the north and east of 

 Padstow, and here it is probable the chief volcanic phenomena 

 of that period were centralized. There can be little doubt that 



*"0n the so-called greenstones of Central and Eastern Cornwall." Quart. 

 Journ. Geol. Soc, Aug. 1878, pp. 30—48. 



