14 Weed and Pirsson — Highwood Mts. Laccoliths. 



would produce the desired result ; we should have to imagine 

 selective melting and absorption, a quite incredible idea. 



Moreover this is shown by a consideration of the chemical 

 character of the two main types of igneous rocks. An analysis 

 of the shonkinite has been made for us by Dr. W. F. Hille- 

 brand in the laboratory of the U. S. Geological Survey and is 

 given below under ISTo. 1. Dr. Hillebrand has also made a 

 partial analysis of the syenite, the character of the material not 

 warranting more extended work, and this is given under -No. 2. 



Analyses. 



I. II. 



Si0 2 47'88 50-00 



A1 2 3 12-10 19-36 (+Ti0 2 + P 2 5 ) 



Fe 2 3 3-53 3-87 



FeO ... 4-80 2-67 



MgO . 8-64 2-18 



CaO ____ 935 4-96 



Na 2 2-94 3-63 



K 2 .... 5-61 8-52 



H 2 O + 110° 1-52 3-53 (Ign.) 



H 2 O-110° -70 -46 



C0 2 -12 



Ti0 2 -77 with A1 2 3 



Zr0 2 ..... -03 



P 2 5 _. 1-11 withAl 2 3 



V 2 3 -04 



S0 3 none 



CI tr. 



F. ..„_ -05 



S_. -02,5 



Cr 2 3 -035 



NiO._ tr. 



MnO -15 



BaO -46 



SrO._ _ -13 



Li 2 none .... 



99-99 99-18 



The analysis of the syenite shows 12 per cent of alkalis and 

 in order to change this to the composition of the shonkinite an 

 enormous amount of material would have to be added. The 

 same may be said with regard to the alumina. It will be 

 noticed, however, that the silica remains practically the same. 

 The argument is the same as that which we have already 

 adduced for Castle Mountain* and at Yogo Peak in the Little 



Belt Mts.f 



&' 



* Bull. U. S. Geological Survey, No. 139, 1896, p. 133. 

 f 20th Ann. Rep. U. S. Geol. Surv., 1900, Part 3, p. 577. 



