182 BULLETIN OF THE 



It does not seem possible that the contrast of clouded kaolinized core 

 and clear rim could be due to selective decomposition acting on one 

 homogeneous grain, by which the centre was attacked, while the rim 

 was left clear. The clastic outline of the kaolinized portion, the in- 

 clusions of mica, quartz, etc. in the clear part, showing a difference of 

 origin, and the intricate manner in which the clear portion sometimes 

 ramifies through the core, seem to negative this supposition. 



It seems necessary to regard the kaolinization as antecedent to the 

 formation of the clear feldspar. 



What then is the I'elation between core and rim 1 In all the cases 

 described in this paper, the two parts extinguish together, aud, as in 

 the cases described by Van Hise, seem to be crystallograjijhically and 

 optically continuous. Twinning sometimes runs from one to the other, 

 sometimes dies out in the clear feldspar. In many cases, however, the 

 polarization tint of the core is different from that of the rim, and ob- 

 served with the highest powers this seems to be inherent in the feldspar 

 of the core and not due to the visible products of kaolinization ; in cases 

 such as that of Figures 5 and G the kaolinized portions are simply little 

 areas which grade imperceptibly into the clear feldspar, which perme- 

 ates them in every direction. It does not seem possible to explain all 

 these cases by mere outward growth of the feldspar grain by addition of 

 fresh feldspar of the same species to the core ; but rather by an actual 

 replacement of the detrital core by the feldspar of the enlargement, 

 which even in the least advanced stage in this rock has gone so far as 

 to leave but little beyond traces of the original feldspar, and the kaoim- 

 ization products. The whole series by which, if this exphmation is the 

 true one, even the kaolinization products are seen gradually to dis- 

 appear, could only be described and figured here by isolated cases, 

 although there is an intimate gradation in the slides. The smaller feld- 

 spars generally show this absorption most completely, until the per- 

 fectly clear type of Figure 8 is reached, which is undoubtedly albite. 

 The feldspar of the intermediate cases has entirely similar properties 

 aside from the presence of tlie fluid inclusions, kaolin, and iron oxide. 

 In some of the larger feldspars the core is microcline, and the rim is 

 perhaps the same (No. A) and this rim encloses small cr^'stals of albitic 

 feldspar and is adjacent to glassy microcline, which even penetrates the 

 whole grain ; therefore it is necessary to suppose a partial replacement of 

 the original grain by microcline (or regeneration if the clastic grain itself 

 was originally microcline), with enlargement and subsequent formation of 

 microcline outside the whole enlarged grain, which had been fissured. 



