Wadsworth.] 
290 
[November 2, 
around the larger felds]3ar crystals. These latter crystals are 
tabular form, and in the specimens collected vary from two to 
twelve millimeters in length. They are frequently rounded and 
sometimes penetrated by the groundmass. 
Under the microscope the section of No. 1236 is seen to be 
composed of an isabel yellow and dirty white base holding tabu- 
lar and ledge-formed feldspar crystals. The base has decomposed 
to a kaolinized globulitic mass showing feeble aggregate polariza- 
tion. This globulitic structure is the product of alteration and is 
entirely distinct from the original globulitic base of the unaltered 
basalts. The structure of the base is such, that taken in connec- 
tion with the relations of the feldspars to it and to one another, 
it is probable that it was originally a glass. Such, too, would be 
its probable character, from its position at the base of the flow. 
A few dark grains and microlites were seen scattered here and 
there through the base. 
The feldspars of the groundmass showed no polarization, 
other than that belonging to monoclinic crystals ; although it is 
not improbable, that some of the ledge-formed crystals are tri- 
clinic. Most of the feldspars are seen to be in simple twins. They 
contain patches of the base, and a few colorless and dark colored 
microlites. 
One of the feldspars, porphyritically inclosed in the ground- 
mass, was obtained in the section. This is clear, glassy, and con- 
tains only a slight amount of the groundmass and a few full fluid . 
cavities. It is a simple crystal of sanidin. Two of the feld- 
spar crystals are seen to be decomposed into a kaolinized feebly 
polarizing mass, — pinite of Crosby. 
In some of the hand specimens a banding of lighter and darker 
shades is observable. This shows in the more altered specimens, 
and is not owing to difference in mineral structure or composition ; 
since the smaller feldspars extend from one band into another, while 
the fluidal lines do not correspond to the banding. The banding 
of colors is probably connected with the decomposition to which 
the rock has been subjected (1243, 1239). 
The more highly altered portions, especially towards the surface, 
break into thin laminae, more or less parallel with the original 
base of the flow (1240, 1242). This platy structure, as well as the 
color bands, have apparently been taken for stratification planes. 
