Pi7'sso7i and Washington — Geology of Bed Sill, JV. Tl. 275 



The study of the sections also convinces one that there is 

 less of lenad minerals, nephelite and sodalite, than in the fore- 

 going type, an impression confirmed by the calculation of the 

 norm. 



Bayley states that there is about 80 per cent of feldspars 

 and 15 per cent of hornblende, etc. in the rock, and the norm 

 confirms this. We may then say that the mode of the rock is 

 made up of the following minerals in the proportions indicated. 



Orthoclase 32 



Albite _ _ . 43 



Nephelite 7 



Sodalite 2 



Hornblende 12 



Accessories . 4 



Total 100 



The accessories include biotite, diopside, segirite, magnetite, 

 titanite, zircon, apatite, etc. 



The texture of this type is prevailingly trachytoid, with 

 feldspars of a more or less pronounced flat, tabular form. 

 No sharp line between this and the foregoing type can be 

 drawn in the field from the exposures. And among the loose 

 blocks transitional forms of every degree may be found. 

 Hand-specimens may be prepared showing the coarse granitoid 

 texture at one end merging quickly into a quite fine trachytoid 

 one at the other. Such transitions may also be observed in 

 place, and it was thought at first that the trachytoid type 

 occurred in dikes in the other variety, but there are no evident 

 contacts to be seen and it is much more probable that the one 

 type passes irregularly into the other. 



Chemical Composition. — This is shown by Hillebrand's 

 analysis quoted from Bayley, which has been given in the 

 foregoing table. It will be seen that it differs from the pre- 

 ceding type in containing a little more silica, considerably 

 more lime, iron and magnesia and less alumina and alkalies. 

 It is of interest to note that the proportion of Na 2 : K 2 is 

 0-46 in No. I and 0*49 in No. II, practically the same relation. 

 Also that the alumina (molecular ratios) divided by the alka- 

 lies gives l - 02 in No. I and 1*05 in No. II. The change then 

 is in the increase of lime, iron and magnesia and a little silica; 

 there has been no other marked differentiation. 



Classification. — The calculation of the norm from Hille- 

 brand's analysis is given below in No. II, and the norm of No. 

 I is repeated for comparison. 



