ransome: albitite dikes 115 



The rock is of almost snowy whiteness and consists chiefly of 

 striated feldspar with minute, dark, disseminated crystals whose 

 form and luster as seen under a hand lens are at once suggestive 

 of anatase or rutile. The general texture of the rock is equi- 

 granular, the average diameter of the grains being 5 mm. or less. 



In thin section under the microscope the rock appears as essen- 

 tially an aggregate of subhedral and much-twinned plagioclase. 

 Some of the angular spaces between the feldspar are filled with 

 clear calcite, others with a fine-grained, foliated aggregate con- 

 sisting chiefly of kaolinite, and still others with quartz. 



The feldspar, which is generally fresh, has a refractive index 

 of about 1.54, a birefringence equal to or lower than the quartz 

 in the same section, an extinction angle on (001) of about 4° and 

 an observed maximum extinction in the zone normal to (010) of 

 19°. It thus belongs at the albite end of the plagioclase series 

 although it contains a small proportion of the anorthite molecule. 

 The twinning lamellae according to the albite and pericline laws, 

 are generally thin, irregular and discontinuous, so that some of 

 the crystals as seen in thin section have almost the plaid-like 

 appearance of microcline. Carlsbad twins are rare and the crys- 

 tals are not noticeably zoned. 



The calcite areas as seen under the microscope are bounded in 

 part by the crystallographic outlines of the albite and in part pene- 

 trate the contiguous fresh feldspar in a manner indicative of re- 

 placement. Without much doubt the calcite is secondary al- 

 though it does not appear to have been formed by the local de- 

 composition of feldspar but rather to have been introduced by 

 solutions into miarolitic cavities which to some extent have been 

 enlarged by metasomatism. The similarly interstitial kaolinite 

 likewise shows no evidence of being a product of decomposition 

 in situ and the adjacent feldspar is not kaolinized. The kaolinite 

 also may have been carried into small cavities by infiltration. 

 Finally, the quartz, which is not a conspicuous constituent and 

 is not distinguishable at a glance from such sections of albite 

 as show no twinning, may in part be of secondar}^ introduction; 

 for some of it displays the shadowy extinction and the suggestion 

 of plumose structure that are specially characteristic of quartz 

 deposited in veins. 



