1881 .] 
291 
[Wadsworth. 
In a further stage of decomposition the rock is discolored by 
yellow ferruginous spots, and the larger feldspars become kaoli- 
nized or else changed into a soft greenish mass (pinite, Crosby). 
The undecomposed larger feldspars are no longer glassy sanidins, 
but distinct and well marked orthoclase crystals, thereby proving 
in this case, that the orthoclase is the somewhat altered state of 
the former (1244). The color of the surface specimens is a light 
brownish and yellowish gray, weathering yellowish. 
In the section from ISTo. 1243, the base is altered to a mixed 
fibrous and globulitic mass, holding irregular opacite masses and 
fibres. The feldspars of the groundmass are chiefly ledge-formed 
simple twins showing fluidal structure, and are considerably 
decomposed. Many of these, as well as the base, show aggregate 
polarization. Secondary quartz in irregular masses also occurs. 
Similar to the last is the section from No. 1242, but the base is 
less in amount, lighter colored, and more highly altered ; also fer- 
rite masses and fibres are abundant. Biotite laminae were seen 
in the hand specimen. In another section the alteration has pro- 
gressed so far, that an apparent blending of the edges of the feld- 
spar crystals with the base has taken place, so that the former 
show well defined outlines in places only. Also much secondary 
quartz was observed, although the section is nearly free from 
other products of decomposition, except the kaolinized material. 
The porphyritically inclosed feldspars are clear, glassy, simple 
crystals of sanidin. 
The extreme decomposition of the rock yields yellow ferrugin- 
ous, and grayish white clays. 
It seems probable from the macroscopical and microscopical char- 
acters that originally the rock was composed of a glassy or nearly 
glassy base, holding feldspar crystals, and nearly free from other 
interpositions. This groundmass held porphyritically inclosed in it 
feldspar (sanidin) crystals and some biotite. Its simplicity of 
mineral structure is very remarkable, for all the quartz, opacite, 
and ferrite are evidently the products of decomposition. 
The writer has taken great pains to ascertain the relations of 
this trachyte to the felsite and the felsitic debris (porodite), 
spending many hours in digging upon it and exposing fresh 
surfaces. He has tried to be sure of his ground at every step, for, 
