380 Haworth on the Archcean Geology of Missouri. 
rocks is quite interesting. Instead of appearing to be a glassy 
groundmass in which the crystals are imbedded it looks more 
as though the coarse crystals formed first and crowded the glass 
into the interstitial spaces, so that the structure is that of a typi- 
cal diabase in which a portion of the augite is replaced by the 
glass. This jDarticular type of structure is best illustrated in 
number 390, and in those from the " Tin Mines," numbers 359^ 
411, 414, and 415. Fig. 6, pi. i is taken from number 415, and 
shows a portion of the section as it appeared in the field of view 
when magnified 44 diameters. The occurrence of so much 
glass in such a coarse-grained rock is rather unusual. 
It is also interesting to note that at the " Tin Mines " the 
amount of glass decreases as we descend below the surface. 
Thedyke cuts through a big hill of porphyry fully 400 feet high» 
Years ago four different tunnels were driven into the hillside 
in a vain search for tin. The first of these is about 75 feet 
above the base of the hill; the others are higher wp. Number 
416, from this first tunnel, shows no glass, 415 has a small 
amount relatively, while 414 and 411 have a great deal of it.. 
This locality was not visited by the writer, but from the char- 
acter of the specimens received from Mr. Payne it would seem 
that we here have an excellent illustration of the crystallization 
of a molten magma being dependent upon the depth below the 
surface at which it solidifies. 
The structure of the ordinary diabase and olivine diabase is sO' 
well known it is useless to give it here in detail. 
(c). Classijicatioii of the diabases. 
From what has already been given it will be seen that with 
reference to their mineralogical composition we have three 
grades, or classes of diabases, the quartz-bearing, the olivine- 
bearing, and those with neither of these minerals; also that the 
last division maj be sub-divided into the uralitized diabases, and 
those in which the augite is not altered. With reference to the 
structure we also have a number of varieties. There are the 
holocrystalline, hypidiomorphic ones; the glass-bearing, coarse- 
grained, hypidiomorphic ones, and those with a true poi-phyritic 
structure. Only one j^orphyritic specimen was found which 
contained olivine, but further search woidd probably reveal 
