626 



A TREATISE ON METAMORPHISM. 



Fig. 14.— Enlargement of feldspar 

 fragment. 



be deposited in optical orientation upon nuclei of like minerals. Feldspar 

 enlargements have been found in many arkoses. Perhaps the best known 

 instance is that of the Keweenawan sandstone of Lake Superior, the cemen- 

 tation of which is mainly accomplished by the deposition of feldspar upon 

 worn grains of that mineral." The grains are of 

 different kinds of feldspar, orthoclase, and various 

 plagioclases. The material deposited in each case 

 is in optical continuity with the old material, even 

 to the extension of the twinning lamella?. (See 

 fig. 14.) An excellent instance of the enlarge- 

 ment of hornblende is that of the hornblende 

 crystals of the volcanic tuffs of Kekekabic Lake, 

 in northeastern Minnesota. 6 The relations of the 

 cores and additions of hornblende are identical 

 with those of the feldspars, even to the extension 

 of the twinning lamellae. (See fig. 15.) 



The enlargement of feldspar and hornblende 

 furnish the best illustrations known of the principle so strongly emphasized 

 on pages 120-122, that mineral nuclei already present are able to abstract 

 from solutions materials like themselves, and thus control 

 the combinations of the elements when precipitated. In 

 this principle probably lies the partial answer to the ques- 

 tion why, in the cases cited of feldspar and hornblende 

 enlargements, the material was not deposited between the 

 grains as hydrous minerals. The formation of hydrous 

 compounds would have developed more heat, but the 

 power of the nuclei of old minerals to control the precipi- 

 tation appears to more than have overbalanced the chemi- 

 cal law that in the belt of cementation reactions commonly 

 take place which liberate the greatest amount of heat. 

 But another factor which frequently enters into the precipitation of such 

 anhydrous minerals as feldspar, hornblende, and mica is the abundance 

 and the proper proportions in the solutions of the elements out of 

 which they can be made. Rocks in which feldspar, hornblende, etc., 



a Van Hise, C. R., Enlargement of feldspar fragments in certain Keweenawan sandstones: Bull. 

 U. S. Geol. Survey No. 8, 1S84, pt. 2, pp. 44-47. 



>' Van Hise, C. R., Enlargements of hornblende fragments: Am. Jour. Sci., 3d ser., vol. 30, 1885, 

 pp. 231-235. 



Pig. 15.— Enlargement of 

 hornblende fragment. 



