SURROUNDING THE LAND's-END MASS OF GRANITE. 417 



chlorite; the clear quartz is crowded with fine crystals of both 

 minerals ; and, in addition to individual crystals scattered through 

 the mass, numbers of small tourmalines are collected together and 

 form most beautiful microscopic groups of radiating prisms. Being 

 set in clear colourless quartz, they are exquisite objects in polarized 

 light. 



Alteration. — Many of the rocks here described contain excellent 

 examples of the alteration of tourmaline, and the process may be 

 observed in various stages. 



Chloritic pseudomorphs are formed by the gradual conversion of 

 a crystal into a green substance which exhibits no bright colours, 

 but shows the distinctive characters of aggregate polarization, and 

 under a magnifying power of 200 is seen to consist of a mass of 

 confused fibrous bundles. In other cases the fibrous structure is 

 indistinct, and there are vermicular crystals in various stages of 

 formation. 



Other pseudomorphs appear to be micaceous ; some of the plates 

 exhibit distinct cleavage-lines, and they are slightly dichroic, chan- 

 ging from a light yellowish-green to a darker shade ; the absorption 

 of light, however, is considerable. 



These pale green substances show very clearly how greatly the 

 absorptive property of minerals depends on the dissemination through 

 them of minute quantities of colouring-matter : many of them con- 

 tain extremely minute black grains, which form the nuclei of indi- 

 stinct spots of a barely perceptibly darker shade than the light yellow 

 parts ; the shade deepens, however, as the polarizer is rotated, and 

 they gradually become dark opaque spots, devoid of any well- 

 defined edge, but strikingly different from the surrounding parts. 



Liihia Mica. — In the altered slates and granite there are three 

 varieties of mica, which may be readily distinguished from each other 

 under the microscope ; they occur, however, in such small flakes that 

 it is impossible to obtain sufficient for chemical analysis or optical 

 examination in the usual way. 1. A dark brown mica, resembling 

 biotite in its action on polarized light, changing from light yellowish- 

 brown to very dark brown, and nearly opaque. 2. Red mica, which 

 also exhibits dichroism and absorption, but to a far less degree than 

 the preceding. 3. White mica, in minute scales, having a pearly 

 lustre. I have examined the three varieties by the following me- 

 thod : — A few small flakes were carefully collected, moistened with 

 fluoric acid, and placed on a platinum wire, then exposed to the 

 flame of a Bunsen's burner and examined with the spectroscope. 

 The red lithium-line was quite distinct in all three, but flashed out 

 more brightly and remained longer in the case of the white mica 

 than in the others, clearly showing a larger proportion of lithium. 

 The white mica is probably a typical lepidolite ; and I think that the 

 difference in the absorptive power of the others is due merely to the 

 presence of a little more ferric oxide in one of them. 



2g 2 



