378 Haworth on the Archccan Geology of Missouri. 
is present, having resulted from the decomposition of some 
of the original. 
In number 366, we have an interesting example of what 
seems to be primary quartz, existing in the form of jDorphyritic 
individuals. This rock, which came from sec. 10, T. 33. N. 
R.V. E., is a diabase porphyryte with a tolerably fine grained, 
almost holocrystalline groundmass, consisting essentialy of pla- 
gioclase and augite. It has numerous large, rounded, porphy- 
ritic feldspars, some of which are two centimetres in diameter. 
The porphyritic quartz is much smaller, the average diameter 
being about two millimetres. Fig. 5, a. pi. i, represents one of 
these quartz-graines, and c, of the same figure is a secondaiy 
quartz. 
Around these supposed primary quartz grains there is a 
border .oS mm wide which apparently is the result of a partial 
refusion of the quartz grain by the magma just before its final 
solidification. The border is composed almost entirely of little 
slender augite crystals a majority of which are radially arranged 
around the quartz. Some of these are as long as the border is 
wide, and some of them are much shorter. The inner portion 
of the border is decidedly green, but the color fades towards 
the outside, even in the same augite individual. 
Mr. Charles E. Coates, graduate student in the chemical 
department of the Johns Hopkins University kindly made a 
silica determination of this rock for me which showed it to con- 
tain 53.4 per cent. SiO ^ . It is by no means common to find 
free silica in a rock so basic as this. Its existence in such rocks 
which were once molten seems to be a chemical paradox. Re- 
cently, liowever, a number of similar occurrences have been 
reported, one of the most interesting of which is that described ' 
by Mr. J. S. Diller of the U. S. G. S., In the basalt from 
" Cinder Cone " near Lassen peak, in California, he found quartz 
crystals whose presence could reasonably be accounted for only 
on the supposition that they were primary. An analysis of the 
basalt showed it to have 57.35 per cent. SiO^, which makes it 
more acid than the Missouri diabase. Until more is learned 
concerning the effect of heat and pressure upon chemical affinity 
,. 1 Am. Jr. Sci., vol. 33, p. 45, Jan., 1887. 
