Vol. 62.] CATARACTS OF THE RIVEE MADEIRA, ETC. 119 



larger size, measuring occasionally as much as a millimetre in 

 diameter. They are separated by irregular jagoed boundaries. At 

 certain points they show the cross-laniellation characteristic of 

 microcline, but in an ill-defined form ; elsewhere the extinction 

 often occurs in patches and streaks. The index of refraction is 

 lower than that of quartz or albite, and there seems to be little 

 doubt that these felspar-aggregates are to be referred to transitional 

 forms between orthoclase and microcline. In these felspars, the 

 smaller rounded constituents of the rock are embedded as in a glassy 

 matrix, and at the same time recall the enclosures of blebs and 

 rounded crystals of quartz (and occasionally microcline) in the 

 felspars of the rocks of other cataracts. 



An analysis by Mr. G. S. Blake showed the rock to have the 

 following composition : — 



Percentage Molecular 



composition. proportion . 



Si0 2 69-41 • 1-1568 



Ti0 2 0-20 00025 



A1 2 3 1283 0-1258 



Fe 2 3 106 00066 



FeO 2-48 00344 



MnO 012 00017 



MoO 0-71 00177 



CaO 473 00845 



K 5-25 00559 



Na 2 3-13 0-0505 



H 2 above 100° C 0"18 00100 



H 2 Oatl00° C 011 



Total 100-21 



It belongs to the subrang adamellose of the American classifi- 

 cation, and corresponds in chemical composition to the alkali- 

 granites of ordinary nomenclature. 



(5) The Granulites and their Genesis. 



It will be seen that, in one way or another, the great majority of 

 the rocks of the cataracts are characterized by the presence of the 

 small rounded crystals or blebs which are typical of the granuiites. 

 But the extent to which this structure is developed varies consider- 

 ably from point to point. In some cases it prevails to the exclusion 

 of any other, while in others it is only characteristic of a portion, 

 sometimes but a small portion of the rock, the rest of which is 

 formed by larger and unrounded crystals such as are found in a 

 normally-developed holocrystalline rock. These appear to have 

 been formed at a subsequent date, when the crystallization of the 

 semi-consolidated rock continued and was completed under changed 



