Prof. Stokes on a Theory of the Composition of Colours. 431 

 Table showing the Relative Compression of Cleaved Rocks. 



II 





II 



:2 2 



1% 



g . 





Intersection of cleavage ' 

 and bedding 



Dip of cleavage 



Perpendicular to clea-' 

 vage 



1-000 

 1026 

 2-426 



1-000 

 0-990 



1-000 

 1-494 



3-889: 9-839 



1000 

 1000 

 6-881 



1-000 

 1-195 

 1-881 



1000 

 1-000 

 3-697 



1000 



1-000 



11-105 



1-000 

 1-000 

 2-144 



The preceding Table is, I believe, the first attempt to estimate 

 numerically the amount of compression necessary to produce 

 cleavage in rocks ; and it is to be hoped that the attention of 

 geological observers may in future be more directed to the obser- 

 vation of the relative position of the planes of cleavage and bed-* 

 ding, and of the distortion of the fossils : such observations, to 

 be of any value, should be made in the field, by persons com- 

 petent to judge of the shape of the fossil forms, and carefully 

 instructed as to the precise points necessary to be observed. 



An opportunity of obtaining the information requisite for a 

 perfect mechanical theory of cleavage is afforded by the means 

 of observation possessed by the Staff of the Geological Survey of 

 Great Britain and Ireland. If the operations of such a survey 

 were conducted with the science and skill necessary for carrying 

 out such an important work, nothing would be wanting to com- 

 plete the data requisite for the solution of many interesting pro- 

 blems in physical geology. But it is much to be feared, that so 

 long as geologists remain in the half-educated condition with 

 which they are at present contented, that such an amount of 

 intelligent observation is not to be looked for; and that those 

 who wish for satisfactory foundations of facts on which to build 

 their theories, must even be content to take their hammers in 

 their hands, and having strapped on their knapsacks, to seek in 

 the field of nature the facts for themselves. 



Trinity College, Dublin, 

 October 20, 1856. 



LIII. Remarks on Professor Challis's paper , entitled " A Theory 

 of the Composition of Colours/' &^c. By Professor G. G. 

 Stokes*. 



MY object in the present communication is not to discuss 

 Professor Challis's theory, but to rectify some statements 

 as to the experimental facts of the case, as well as one relating 

 * Communicated by the Author, 



