102 CONTRIBUTIONS TO MINERALOGY. [bull. 262 
is a>Xi=C. Optically it is negative, the acute bisectrix a bein|g 
normal to the base. The axial plane is parallel to the brachypinacoid 
The birefringence is higher than that of quartz, though only by i 
small amount. 
The sections of dumortierite show that inclusions are rare. Tlu 
masses of mineral are usually pure, muscovite being about the only 
mineral included, and this is not strictly an inclusion, as it is g 
secondary product, forming along the fibrous cracks of the mineral. 
In one basal section a number of rutile (?) crystals were observed, but 
such inclusions are very rare. 
The quartz occurs in large allotriomorphic grains, but little cracked 
and sometimes almost free from inclusions and again thickly crowded 
with them. In no way does the quartz show the effect of pressure as 
the quartz of the upper part of the dike does. Occasionally when the 
nicols are crossed the color of a quartz grain will be somewhat 
banded, these bands occurring on several grains, being parallel 
throughout the entire slide. The inclusions are sillimanite, magnetite, 
rutile (?), and an undetermined mineral. 
The rock composing the dike is doubtless an igneous plutonic rock. 
There would seem to have been an interruption in the stage of forma- 
tion of the entire dike. The upper or sillimanite-bearing part was 
more or less completely formed when a mass was pushed into the place 
now occupied by the lower part of the dike. This mass exerted a 
great pressure on the already partly formed upper part, giving it its 
schistose structure. Boric acid in large amount Avas in the lower 
mass. Possibly it was a later intrusion and may have caused the 
increase in pressure of the lower mass, which gradually cooled and 
formed a borosilicate (dumortierite) and quartz. The absence of 
dumortierite in the upper part would seem to indicate that the addi- 
tion of boric acid to the entire mass was a later incident, and had the 
original magna cooled without this extra intrusion the rock would 
have been uniform and would have consisted of an aluminum silicate 
(sillimanite or andalusite) and quartz. The intrusion of the boric acid 
could not change the character of the upper part, which had already 
(at least partially) formed, but it did change the character of the 
lower part. May it be that the pressure determined the formation of 
sillimanite instead of andalusite? 
WASHINGTON. 
Ford mentions this occurrence of dumortierite and gives as the 
locality "the headwaters of the North Fork of the Washougal River 
in Skamania County, Wash." Mr. Brereton, of Woodstock, Oreg., 
very kindly sent the writer a number of specimens, which form the 
basis of the following notes. 
As Ford states, the dumortierite occurs in a new habit, being present 
as small spherulites up to 3 mm. in diameter. 
