416 



ME. J. PAEKTNSON ON" THE GEOLOGY OF [Aug. 1903, 



Lesnewth from Waterpit Down. 



with a 1-inch objective fifty or more may be found in the field at 

 the same time. Frequently the form is granular, without indication 

 of crystalline faces, but acute-angled rhombs are not uncommon. 

 The mineral is earlier in formation than the epidote. In the latter 

 cleavage is often well marked, the crystals are traversed by numerous 

 cracks, and are occasionally twinned : the pleochroism is feeble. 



In these slides the ilmenite and sphene form distinct and separate 

 bands, parallel with the general foliation of the rock. It is inferred 

 that the titanium-oxide present in the original rock has been 

 utilized to form either sphene or ilmenite, according to the local 

 presence or absence of lime. Granules of sphene may often be seen 

 clinging to the ilmenite-grains. 



The epidote from some of the rocks hereabouts and from the 

 adjacent South- Western Railway-cutting (No. 91) contains crystals 

 of allanite l (fig. 2) . The crystalline faces of the allanite are parallel 



to those of the enclosing 



Fig. 2. — Crystal of epidote, enclosing allan- mineral, but the orien- 



ite and embayed by calcite (c), x Jfi. tation is different, for 



South of Tregrylls Farm, on the road to the greater length of 



the allanite is usually 

 oblique to the greater 

 length of the epidote. 

 Cleavage is absent, the 

 index of refraction is 

 higher than for epidote, 

 the double refraction 

 is considerable, zonal 

 structure does not oc- 

 cur, but the mineral is 

 often rather irregularly 

 coloured. The scheme 

 of pleochroism is diffi- 

 cult to determine, but 

 the colour varies from 

 reddish-brown to pale 

 yellowish-brown, often 

 with a greenish tinge. Extinction takes place parallel to the greater 

 length of the crystal. 



An allied rock from Trehane, near Davidstow Church, contains 

 crystals of tremolite in a ' base ' of chlorite and actinolite-nakes ; 

 while another from the eastern side of the mouth of the Rocky 

 Yalley contains rounded or rectangular translucent felspars up to 

 a tenth of an inch in diameter, and occurring rather as do the 

 garnets of some schists. These felspars appear to be original (that 

 is, not authigenous) constituents which have undergone reconstruc- 

 tion in place. In the chloritic ' base ' of the rock are developed a few 

 elongated crystals of secondary hornblende. The pleochroism of 

 this mineral varies from a deep green, very slightly tinged with blue, 

 to a pale yellowish-green. Opacite, incipient sphene, occasional 



1 Compare W. H. Hobbs, Am. Journ. Sci. ser. 3, vol. xxxviii (1889) p. 223. 



