8o PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



as a rule, those which originally contained the most augite or 

 olivine in the ash, as shown by the pseudomorphs. 



Fig, 7. — Fragment of pumice Fig. 8. — Small concretion in the Slate 

 filled luith a green mineral, of Birnam. 



I have endeavoured to find out the true relations of all the dif- 

 ferent green or analogous colourless minerals so often developed 

 in slates since their deposition; but though it is quite easy to 

 distinguish several, and to recognize each well enough in other 

 specimens, I must say I feel much hesitation in identifying them 

 with known minerals. We should not, however, be far wrong if 

 we regarded the whole group as evidence of the same general 

 physical conditions, and the production of each particular kind as 

 evidence of some difference in the chemical composition of the 

 material. This appears to me well established by what may be 

 seen in some of the fine-grained slates. Thus, for example, in the 

 slate of Birnam, near Dunkeld, small elongated concretions have 

 been formed along the planes of cleavage, as shown (magn. 150 

 linear) by fig. 8. These are composed of alternations of two 

 very distinct laminar minerals. One is colourless, with a de- 

 polarizing power several times greater than that of quartz, the 

 negative axis being perpendicular to the laminae ; whilst the other 

 is deep green, very dichroic, has a very weak depolarizing power, 

 and gives with polarized light a remarkable copper- colour, not cor- 

 responding with any interference tint. In the slate of Penrhyn we 

 meet with similar but shorter concretions, composed of a mixture 

 of the above-named colourless mineral (which may be talc) with 

 one that is green and dichroic, with a depolarizing power much 

 weaker than that of quartz, having the positive axis perpendicular 

 to the laminae, and further differing from the analogous mineral in 

 the Birnam slates in giving perfectly normal though low inter- 

 ference tints. According to Dana, similar mixtures are met with 

 in larger crystals of minerals belonging to the same general group 

 as those now under consideration. I presume that we shall all 

 agree that the production of such minerals indicates the more or 

 less prolonged action of water. I do not see any need to assume a 

 high temperature ; but I must say that it appears to me that their 

 production is evidence of the preexistence of material corresponding 

 to more or less decomposed augite, hornblende, or olivine ; and I 

 think that the presence or absence of such products depended more 

 upon the original nature of the material than upon any thing else. 



