454 
IDDINGS. 
mony with the observation that the micropegmatitic struct¬ 
ure and the other phases of crystallization in this part of the 
rock are irregularly scattered in patches, so that adjoining 
parts of adjacent spherulites are micropegmatitic, wdiile other 
portion’s of them are free from this structure. Such crescent¬ 
shaped spaces in the spherulites of this rock have undoubt¬ 
edly been produced at the time of the crystallization of the 
feldspar rays by conditions which affected the quantity or 
distribution of the free silica or of the mineralizing agencies 
engaged in its crystallization. 
In parts of the rock the small feathery spherulites 
bordering an area of tridymite do not terminate in well-de¬ 
fined feldspar prisms, but pass out into irregularly shaped 
feldspars and send out acicular rays of extreme delicacy. 
These transparent needles also lie in various directions in the 
tridymite area. They have apparently the same double re¬ 
fraction as orthoclase, and have the axis of greatest elasticity, 
IX, parallel to their length. A transverse parting is slightly 
developed. Their mineralogical character cannot be made 
out with any degree of satisfaction from the smallest needles, 
but they can be traced to stouter ones which are undoubt¬ 
edly orthoclase; so that there would seem to be no question 
that the delicate acicular rays of these spherulites are needles 
of orthoclase elongated parallel to the clinoaxis. They are 
then the same as the stouter prismatic rays, but have prob¬ 
ably developed an acicular form because of some slight dif¬ 
ference in the conditions under which they crystallized. 
The spaces between the spherulites already mentioned are 
in most instances occupied by tridymite in comparatively 
large crystals, often twinned and carrying numerous gas 
cavities. These patches may be completely filled with 
tridymite, or may be but partially filled, there being open 
spaces between the crystals which cross one another in all 
directions, and in extreme cases the tridymite may simply 
coat the walls of a hollow cavity. In most instances the 
area is completely occupied, when the free silica is sometimes 
in the form of quartz. There are always other minerals 
