17 



No. I consists of microcline and plagioclase, with 

 small amounts of quartz and of an untwinned feldspar, 

 as well as very subordinate amount of biotite and horn- 

 blende. It is impossible in the case of this analysis to 

 calculate the exact proportions of the iron-magnesia con- 

 stituents which are present, on account of the fact that 

 the exact composition of these minerals is not known. 

 The 'norm,' however, is given below. By this is meant 

 the calculation of the analysis into the form of certain 

 standard minerals, showing the mineralogical composition 

 of a rock into which such a magma might crystallize under 

 slightly different conditions [3, p. 47]. The norm repre- 

 sents in this case very nearly the true percentage of the 

 various minerals present, although diopside and hypers- 

 thene are calculated as present, which are represented by 

 other combinations in the actual rock. It is as follows: — 



Orthoclase 18-35 



Albite 42-44 



Anorthite 5-28 



Quartz 27-72 



Diopside 2-57 



Hypersthene 1-92 



Magnetite 0-93 



Ilmenite 0-30 



Total 99-5I 



In this rock the albite and anorthite shown in the 

 analysis are combined in an acid plagioclase which exceeds 

 in amount the potash-feldspar, a fact which is also shown 

 by separations with Thoulet's solution. The rock takes 

 its place in the quantitative classification as a lassenose. 



No. II is seen under the microscope to be composed 

 essentially of feldspar and quartz, with a small amount of 

 biotite. Untwinned feldspars, apparently orthoclase and 

 microcline, are present in large amount, and the proportion 

 of feldspar grains showing the ordinary albite-twinning is 

 small. A little iron ore and a very few minute individuals 

 of apatite and zircon complete the list of constituents 

 present. A separation of the constituents of the rock by 

 means of Thoulet's solution showed that the amount of 

 oligoclase present was considerably greater than that of 

 orthoclase and microcline taken together, a conclusion 

 confirmed by the analysis. This, when calculated out, 



35064—2 



