92 
The American Geologist. 
February, 1&9& 
in Missouri; and Archman rocks are present which are not unlike those 
of the more typical areas in other parts of the American continent. 
T^is being the case, the normal and unchanged granitic rocks of the St.. 
Francois region probably do not belong to the Archaean at all, but to 
that recently established system which represents the enormous inter¬ 
val of time between the formation of the truly azoic rocks and the dep¬ 
osition of the lowest Cambrian. 
Recapitulating briefly, it may be stated that: 
1. The granites and porphyries are very closely related genetically, 
and are to be regarded as facies of the same magma. 
2. Whatever may have been their origin, whether from a few or many 
points of extravasation, the present relations of the two are that the 
porphyry is an upper and surface facies of the granite, the thickness of 
the former being variable, having been originally unequally developed 
in different places and subsequently modified by both ancient and re¬ 
cent erosion. 
3. The present geographic distribution of the granites and porphy¬ 
ries is the outcome of very recent changes in the topographic configura¬ 
tion and is not of very ancient origin, as it appears to have been usually 
regarded. 
4. The existing areal relations of the principal masses of the acid 
rocks may be traced directly to the systematic and widespread physio¬ 
graphic effects of recent orogenic action. 
5. An element of uncertainty regarding the geological age of the 
massive crystalline rocks now prevails, and an exact determination may 
always remain a problem. 
6. The basal complex of Archaean schists exists in the region within 
a very moderate distance beneath the highest Paleozoics. It differs 
widely in lithological characters from the crystallines usually referred 
to that age in the State, but closely approaches the more typical Ar¬ 
chaean rocks of other districts. 
Illustrations of the Dynamic Metamorphism of Anorthosites and re¬ 
lated rock's in the Adirondack's. J. F. Kemp, Columbia College, New 
York City. The high central peaks of the Adirondacks and the larger 
outlying ridges consist of anorthosite, a coarsely crystalline rock that is 
nearly pure labradorite. Though described as norite in earlier reports, 
it is noticeably poor or entirely lacking in ferro-magnesian silicates. In 
the course of a fairly extensive reconnoissance of the principal portion 
of the mountains, the writer has met but a limited exposure of the an 
orthosites in an uncrushed condition. Specimens of such were shown, 
and beginning with these as a starting point the gradual development 
of crushed rims was shown, which, at first barely discernible, increase 
until the original crystals of labradorite are but small nuclei. The ex¬ 
treme is a “pulp-anorthosite,” with no nuclei. The passage into gneis- 
soid forms, through augen-gneisses, and with a rich development of 
o-arnets, was also illustrated. The final result is a thinly laminated 
gneiss. Comments on the areal distribution of these types were added. 
From a series of basic gabbros'a gradual passage was shown, by speci¬ 
mens, into gneissoid types in the same exposure. 
