﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 79 



Now on hearing- of your problem it occurred to me that such an 

 effect might be represented by the bleached dikes in limestone. 

 The idea was that, under the influence of the intrusions, the lime- 

 stone may have become decarbonated to a slight extent, thus 

 facilitating action with the ferric oxid of the feldspar. The ex- 

 planation does not, however, satisfy me, for one might expect 

 perhaps that the feldspathic material intruded at the elevated 

 temperature would have already acted on its iron oxid, and hence 

 not show color; still it may be that the silicate molecule of the 

 feldspar is far more resistant than the zeolite and limestone in 

 respect to ferric oxid, which might thus be in independent exist- 

 ence with the feldspar at high temperatures. 



Dr Warth's article in the Chemical News deals with the blowpipe 

 ignition of mixtures of alumina and ferric oxid in various pro- 

 portions, in which the color invariably changed from red to white 

 when small amounts of iron were used, while a brownish tint was 

 obtained when the proportions were larger. Incidentally it was 

 also shown that the alumina prevented reduction of the iron to the 

 magnetic oxid. 



A sample of finely crushed and sorted red granite was ignited 

 by us for three hours over a Bunsen flame in a platinum crucible. 

 The portion in close contact with the sides and bottom became 

 white, while the bulk of the material, in more central position and 

 hence less strongly heated, retained its red color. This we take to 

 indicate that, with sufficiently high temperature, even in feldspar, 

 the red color will disappear, and that the presence in rocks of 

 alkali feldspar colored red by ferric oxid shows that, under the 

 conditions of congelation, the temperature was not sufficiently high 

 to bring about this color change. We then mixed a small quantity 

 of powdered limestone with another charge of the crushed granite, 

 and ignited in the same crucible over the same burner for the 

 same time. Not only was the feldspar of the entire charge bleached, 

 but the bleaching seemed complete at the end of one hour. Finally 

 we ignited a third charge, in which a very thin coating of lime- 

 stone was spread over the top, but not mixed with the granite as 

 in the previous case, and here again the bleaching was prompt and 

 absolute. It is not intended to imply that the cause of the bleach- 

 ing "Was the same in both cases, but only that, in the presence of 

 lime, decoloration took place more readily and at a lower tempera- 

 ture ; precisely what the field relations had indicated for the granite 

 in place. There is also here, it seems to us, a hint at the reason 

 why red coloration is a common feature in alkali feldspars, and 

 not in lime soda feldspars. 



