34 Diorites and Granites of Swift's Creek, 



They belong to the " endogenous granite veins " of Dr. J. S. 

 Hunt. They, almost without exception, have this cross- 

 banded or twinned variety of sodiferous potash felspar. 



It was, I think, formerly assumed that the triclinic felspars 

 which are constant constituents of the granites, belonged to 

 oligoclase. This also applies to the triclinic felspars of the 

 quartz diorites, their near allies. It was also generally 

 assumed that two or more of the triclinic felspars did not 

 occur together in any one of the igneous rocks. These 

 views have gradually been abandoned. Indeed, if we accept 

 Tschermak's theory that the various felspars, albite, oligo- 

 clase. andesine, labradorite, anorthite, are, in fact, but an 

 iso-morphic series, of which the second, third, and fourth are 

 admixtures of the first and last, there can be no real dif- 

 ficulty in accepting as probable that any two or more of 

 them might occur together, in accordance with the various 

 proportions of potass, soda, or lime in the original magma. 

 Assuming the correctness of Des Cloizeaux's statements, and 

 that the observations which I have made are reliable, we 

 have, then, proof in these Swift's Creek rocks of such simul- 

 taneous occurrence. I have pointed out the general groups 

 into which I have found it possible to collect the felspars, 

 and these groups probably include the whole series. I 

 observe that in the series, orthoclase, microcline, albite, oligo- 

 clase, anorthite — taking these five as an illustration — the more 

 basic the felspars are, the less free quartz is associated with 

 them in the rock. It becomes evident that in the quartz 

 diorites of a very granitic character, microcline in a great 

 measure represents the triclinic forms. It is associated not 

 only with its then subordinate monoclinic form, orthoclase, 

 but also with those felspars, the inclination of whose 

 direction of obscuration to the composition face point to 

 albite or oligoclase rather than labradorite or anorthite. 



All these and other considerations have led me to see that 

 our usual view of the felspars will probably undergo con- 

 siderable modification. We can no longer view them as a 

 group of distinct species, but as forming a series of iso-mor- 

 phic mixtures of certain double silicates of alumina and 

 alkali, or lime, or baryta, in which some particular ratios of 

 the components most frequently occur. It is, in fact, only in 

 accordance with the theory of the felspars as propounded by 

 Tschermak that a comprehensive view of the whole group 

 becomes possible. He has shown that the triclinic felspars 

 are iso-morphic silicates of alumina and alkali, or of alumina 



