ZONAL WEATHERING OF A HORNBLENDE 



203 



rock is practically lost. No microscopical study of this material 

 was undertaken. 



The results of chemical analyses, made in duplicate in every case 

 and in triplicate in a number of determinations, are given in Table I. 

 The following recognized effects of weathering are well illustrated : 

 loss of silica, apparent increase in alumina, increase in ferric iron, 

 decrease in ferrous iron, loss of bases, increase of combined water, 

 and decrease in specific gravity. The very small development of 

 carbonates may be due to the fact that most of the available carbon 

 dioxide has formed soluble bicarbonates in removing lime, mag- 

 nesia, and alkalies. 1 



TABLE I 



* Fresh Rock. 



t Altered near the fresh rock. 



j Altered, most decomposed portion. 



§ Pycnometer method. 



On the assumption of constancy of alumina 2 the analyses may 

 be recalculated, and gains and losses estimated, as shown in Table II. 

 The change in total iron is worthy of note, in that there is a slight 

 loss shown in 2, with a marked increase in 3. A suggested explana- 

 tion is that during the early stages of alteration some soluble ferrous 

 compound was formed, which migrated, perhaps by capillary 

 action, to the outer zone before it was precipitated by oxidation to 



1 In this connection it may be noted that wells in or near the dikes are said to 

 yield water containing considerable amounts of lime, while those in the gneiss, farther 

 from the dikes, yield comparatively soft water. Unfortunately, no quantitative data 

 on these waters are available. 



2 G. P. Merrill, Rocks, Rock Weathering and Soils, p. 208. 



