Minute crystals of iron oxide are seen, apparently in the form of rhom hohedrons, having slightly concave faces. They do not exceed 0.0014 mm. in diameter. The blebs have an average diameter of 0.1 mm. and the pore space has increased to 12.05.
R 3-20—Drawn at cone 9, or approximately 1310°C. Quartz fragments are present as before, but occasionally one is observed the edge of which has fused into a glass. The needle-like crystals are everywhere present in the glass, giving to it the yellowish-green tint before mentioned. The iron ox ides appear much the same as in the last specimen. The blebs are but little changed.
R 3-22—Drawn at cone 11, or approximately 1350°C. The earthy matrix has given place entirely to glass. Quartz particles are still present, but thin; their edges have been rounded by fusion.
The fine needle-like crystals in the glass have increased greatly in length, being in some cases 0.03 mm. long. They exhibit for the first time a marked tendency to collect in radiating clusters. Often they appear to be attached to the corners of the crystals of iron oxide. These latter have increased in number and size, being 0.005 mm. in diameter. In some cases the individ uals unite, forming long serrated columns.
Blebs have increased in size, their average diameter being 0.128 mm. The pore space as estimated from them is 19 per cent.
G 11-10—Drawn at cone 02, or approximately 1110°C. Color, brick red.
As in the R 3 series already described, the mineral fragments consist of quartz, feldspar and mica. Very little glass seems to have developed at this temperature, and no blebs are present. The pore space is made up entirely of pores of the first class, or those due to the imperfect consolidation of the bricks. The average diameter of these pores is 0.065 mm., and the pore
space as calculated is 2.6 per cent.
G 11-12—Drawn at cone 1, or approximately 1150°C. Color red.
A little glass appears. tut no blebs are seen. The average size of pores is lower than in the last slide, being 0.045, but the pore space as estimated runs a little higher, or 3.6 per cent.
It may be remarked that in the slides there is no marked increase in the pore space, as temperature increases, up to the point where blebs appear. From that point on, pore space increases rapidly.
G 11-14—Drawn at cone 3, or approximately 1190°C. Color, reddish brown.
Fine needle-like crystals have formed in the glass. A few blebs appear, but are not in sufficient number to affect the pore space materially. As estimated, is is 3.2 per cent, while the average size of the pores of both classes is 0.06 mm.
G 11-15—Drawn at cone 5, or approximately 1230°C. Color, dark brown.
Quartz fragments are still present, but the feldspar and mica have dis appeared. Glass has formed in great quantity, being colorless, or when acicular crystals are present, greenish yellow. These crystals are present in great numbers and resemble those described in the former' series. Microlites of iron oxide are also present, but have not yet grouped themselves in den dritic forms. Pores other than blebs have disappeared, but the blebs have increased greatly in size, the average diameter being 0.175 mm., while the pore space amounts to 12 per cent.