288 Haworth on the Archccan Geology of Missour 
the rock a mottled appearance, and in fact it is not often en- 
tirely absent. But it never occurs in half the quantity in which 
it is often seen in biotite granites from other localities. • The 
hornblende, when present, is always associated with the biotite, 
occurs in much smaller quantity than the latter, and is generally 
in idiomorphic crystals. These two minerals are quite fresh at 
times, but more often are considerably altered. The alteration 
products are those commonly observed; chlorite, epidote, etc. 
All the other minerals named occur in small quantities, and 
the most of them present no special points of interest. The 
apatite needles are unusually free from the breaks, or fractures 
so common to them in rocks from other places, and when they 
are broken their fragments are not moved out of place, a fact 
in harmony with the field relations which indicate gentle move- 
ments of the rock masses. 
The zircons are very abundant, and are often clustered around 
little fragments of biotite or hornblende, sometimes thirty or 
more being in one group. The dark, pleochroic zones so com- 
monly seen around zircons in granites and other rocks from 
different places, arc very often present in these granites. The 
size of the zircon crystals is not great, the largest one seen 
measuring only o. i iS mm. 
Epidote occurs only as a secondary product from the biotite, 
hornblende, or feldspar. The muscovite is also secondary, and 
occurs in small quantities. The same is true of leucoxene and 
calcite. 
Only those constituent minerals which are especially interest- 
ing will be mentioned in detail. 
(^Ij) Special description of minerals. 
Quartz. The quartz has the limpid appearance so common 
in granitic quartz. The granules vary in size from more than 
a centimeter in diameter to those of microscopic size. There is 
often a 'decided approach to an idiomorphic structure, even in 
the most highly crystalline granites. This is well shown in 
specimen number 303, obtained at Knob Lick as it was being 
loaded on to the cars for shipment. It was said to have come 
from a quarry about two miles north of Syenite. In a thin 
section from this specimen one individual quartz crystal shows 
