ANNIVERSAET ADDEESS OP THE PEESIDEN"T. 5 1 



base of felsites. I do not see how small fragments of felsite 

 could be certainly distinguished from those derived from crystalline 

 felspar, if the former did not reveal a more complex structure, or 

 the latter retain some trace of crystalline form or internal optical 

 structure. 



Minute Calcareous Granules. 



Whilst treating of the character of the minute granules derived 

 from decomposed minerals, or from the complete wearing down of 

 hard quartzose rocks, it will be well to point out how easily we 

 may distinguish from them the calcareous granules derived from 

 decomposed shells or comminuted limestones. Owing to the intense 

 double refraction of calcite, calcareous granules give by depolariza- 

 tion tints of a much higher order than those given by the others. 

 Thus, on an average, grains of j-^^ of an inch diameter give 

 all the series of colours up to the reds and greens ; and even those 

 of iQ QQQ give well-marked tints, commonly the yellow of the 

 first order, but varying up to the red of the first order, according 

 to the manner in which they lie. Moreover, in examining clays 

 deposited in some parts of our country since the Cretaceous period, 

 we may often recognize the coccoliths of the Chalk, not only by 

 their very definite form, but also by the characteristic black cross 

 which each gives with polarized light. 



Pseudomorplis after Felspars. 



As is well known, felspars occur altered to various pseudomorphs, 

 the more important of which, in connexion with my subject, are 

 mainly composed of chlorite, talc, or analogous minerals, which 

 occur as plate-like "crystals, having a very distinct laminar struc- 

 ture, to which the negative axis of depolarization is perpendicular. 

 When these pseudomorphs are seen as thin sections, the laminar 

 crystals cut transversely are much more distinct than those cut 

 nearly in the plane of their cleavage ; and it sometimes requires 

 much attention to distinguish between a mass of such laminae and 

 small prisms scattered about in a different kind of material. Thus, 

 for example, when altered to chlorite or some green variety of talc, 

 the transverse sections are but little coloured, and appear like fibres 

 surrounded by a green uncrystalline base ; but further examination 

 shows that they are very dichroic, and that the green and appa- 

 rently shapeless material is really only laminae Ijdng in the plane 

 of the section, so that the line of vision is nearly in the line of the 

 optic axis. They therefore show the green colour with ordinary 

 light, and no colours due to depolarization when the polarizer and 

 analyzer are used. 



Some portions of what appears to have been felspar have, to a 

 certain extent, been changed into small more or less flattened 

 fibrous crystals, which in many characters approach asbestos, 

 though it is doubtful if they really are that mineral. At all events 

 this seems to be the most probable source of the small fibrous crys- 



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