SURROUNDING THE LAND's-END MASS OE GRANITE. 415 



up bubbles of gas, if one only knew how to recognize them ; there 

 appears, however, to be no reason why gases should not have been 

 mingled with water in various proportions, and have become enclosed 

 in the cavities, so that the latter were never exactly filled with fluid 

 only ; nor is it by any means clear why large cavities should be 

 more likely to contain both than the small ones*. As regards flaws 

 by which fluid may have been lost, it is not likely that an experi- 

 enced observer would be misled by them or by a partial drainage 

 caused by grinding down the sections, as suggested by Mr. Ward. 

 Whatever may be the cause of the discrepancy, it is quite certain, 

 from an examination of a great number of Cornish rocks, that the 

 ratio of the bubble to the cavities is not constant in the same rock, 

 or even in the same crystals, whether the cavities be large or small. 

 This is a fact which may be readily ascertained without making a 

 single measurement by any one moderately experienced in micro- 

 scopic work, and is even apparent in the Tables given by Mr. Ward, 

 and in the figures in his plate xxx. ; it is moreover a fact of pri- 

 mary importance in these investigations,. and appears to me abso- 

 lutely fatal to even an approximate estimation of either temperature 

 or pressure. In conclusion, it can scarcely be necessary to point 

 out that if it be permitted to select such cavities as have a very 

 uniform relative magnitude, and also to assume a standard tempera- 

 ture for the process of crystallization, there can be no difficulty in 

 obtaining concordant results as to the amount of pressure under 

 which the rocks wore formed. 



Moreover, a mean value obtained from a selected number of 

 measurements or by rejecting others that appear to be extreme 

 cases, can obviously be of no value whatever as a basis for exact 

 calculation. 



The supposed exceptions are, in fact, far too numerous to be 

 ignored ; and it will be admitted that no theory as to the origin of 

 these cavities can be satisfactory unless it includes all the facts 

 observed. 



Tourmaline. — The black tourmaline (or schorl) is usually of a rich 

 brown colour as seen in thin slices; but yellow, grey, and blue 

 varieties are not uncommon, and in one specimen of a schorl rock 

 from the Land's End blue crystals (indicolite) are far more abundant 

 than the brown. 



The phenomena of dichroism and absorption are strongly marked, 

 except in the comparatively rare case when a prism happens to be 

 cut at right angles to the principal axis. On rotating the polarizer 

 (the analyzer being removed) the colour changes from a rich brown 

 to pale yellow, and in the blue varieties from blue to a nearly 

 colourless grey. 



Many crystals exhibit a fine blue in one part and brown in 

 another; and not unfrequently the arrangement of the two colours 

 appears to be closely connected with the crystalline form and the 

 mode of growth. This is well seen in PI. XXIII. fig. 7, in which a 



* It must be remembered that all these cavities are of microscopic dimeu= 

 sions. 



Q.J. G. S. No. 128. 2g 



