Haworth on the Archcean Geology of Missouri. 379 
one cannot well sj^eculate regarding the possibility of quartz 
crystallizing out of a basic molten magma. We know that in 
the porphyries both quartz and feldspar are very often partially 
redissolved after being formed, showing that during the effu- 
sive period some cause made the chemical affinity differ from 
w^hat it was at the time these crystals formed. For aught we 
now know^ it is possible that quartz would crystallize out of a 
deejD seated, basic magma in a similar manner, and would be 
wholly or partially redissolved when brought near the surface. 
Glass. About half the specimens examined contain an un- 
crystallized residue. The smaller dykes nearly all have it; and 
some of the larger ones as well. In fact the largest d3'ke in the 
whole district, the one at the place called the "Tin Mines" in 
sec. 30, T. 33. N. R. VI, E., has the most glass. The 50 inch 
dyke on the east side of the St. Francois also has a large amount 
of glass. In the majority of cases devitrification has been 
brought about by the development of small crystals of plagioclase 
so that the original glass has almost disappeared. Trichites and 
crystal skeletons of iron-oxide are very abundant. In some of 
the badly weathered diabase-porphyrytes the former existence 
of a glass base is shown by the presence of these crystal skel- 
etons in the ground mass. 
((5). StriiCiUrc of the diabases. 
The diabases varv in color from black with a waxy lustre, as 
in the case of those from the "Tin Mines," through steel gray, 
to light grey sometimes mottled. Some of them have a greenish 
tinge, and one, number 235, has so much pink feldspar mixed 
with large black augite crystals that it gives the whole rock a 
spotted pink and black color. 
The texture of the diabases varies from coarse-grained with 
crystals two centimetres long to those so fine-grained that their 
macroscopic appearance is that of a homogeneous mass Num- 
ber 284 and 2S9 perhaps are the extremes for fine texture. The 
microscope shows that number 284 is holocrystalline. This is 
quite unexpected, for it came from a six inch dyke in the por- 
phyry. Number 289, from an eighteen inch dyke in granite, 
however, contains a large amount of glass. 
The structure of some of the more coase-graincd glass-))caring 
