114 Transactions of the Academy of Science of St. Louis 
therefore suggested the use of the term felsite as the most suit- 
able general term for these and for intrusive rocks of similar 
type. 
The extrusive rocks include flows of acid to moderately 
basic felsites. Intruded into them are masses of granite and 
granite porphyry. Some of the felsites are also intrusives and 
there are small bosses and sills of diabase as well as basic dikes. 
The age of the igneous rocks is known to be pre-Cambrian 
in most cases, but masses of igneous rocks not seen in contact 
with Paleozoic sedimentary rocks could be of later age. Dikes 
in Ste. Genevieve County cut Cambrian sediments but these 
dikes are of different composition from any of the rocks of the 
main St. Francois Mountains region. It is probable that, with 
the exception of these few basic dikes, all the exposed igneous 
rocks of the state are pre-Cambrian. Though the pre-Cambrian 
of Missouri has not been correlated with that of any other 
region, the lack of metamorphism suggests that the rocks may 
belong to the Algonkian rather than an earlier age. 
PHYSIOGRAPHY 
Topograhy. The topography in the pre-Cambrian area is 
of interest because of its possible relation to structure. It is 
also of importance in explaining the scattered distribution of the 
outcrops. The topography at the beginning of the Cambrian 
period must have been very rugged. Most of the present stream 
courses in the St. Francois Mountains are in pre-Cambrian val- 
leys partly filled with Cambrian sedimentary rocks. In some 
cases valleys are filled to a depth of nearly 1,000 feet with sedi- 
mentary rocks and the felsite hills rise 500 feet or more above 
this level, giving a relief of over 1,500 feet on the pre-Cambrian 
surface. Additional evidence that the stream valleys are pre- 
Cambrian is shown in the fact that the Cambrian sediments are 
inclined towards the center of the valleys because of initial 
dip (4). 
