146 Transactions of the Academy of Science of St. Louis 
post-Cambrian igneous occurrences in the southern portion of 
the Avon block from five to seventy-one and described them as 
“debris choked explosion tubes (diatremes)”. The northwest— 
southeast and northeast—southwest trends of the Lamotte- 
Bonneterre contact are quite striking. A number of faults oc- 
cur along the contact, in some cases bordering projections from 
the main outline of the block. The part of the contact in St. 
Francois County lies outside of the area covered by the Ste. 
Genevieve County map but it is probable that the contact does 
not go far south of this line for the line is approximately on the 
northern border of the Mine La Motte quadrangle and the La- 
motte formation is not shown extending down into that quad- 
rangle. 
The fault indicated east of the county boundary in the 
southern part of the block is in alignment with the indented 
area northwest of it. A large number of post-Cambrian igneous 
occurences appear in groups which show a slight northwest— 
southeast alignment along a line which passes slightly to the 
southwest of Avon (as mapped by Rust(36). This alignment may 
indicate a possible line of pre-Cambrian weakness or faulting. 
The only lead deposit in Ste. Genevieve County that has been 
worked to any extent lies on the southern tip of the projection 
at Avon (49). 
Apparently there was litthe movement of this block as a 
unit, as is shown by the small displacement along the faults. 
However this block, with that of the St. Francois Mountains, 
formed a larger unit with considerable displacement evidenced 
by movement along the faults on the northwest and northeast 
sides of the area. 
Shannon County block. The Shannon County block is lo- 
cated in southeastern Shannon County and northwestern Cartet 
County. It consists of an area of pre-Cambrian felsites form- 
ing a rectangle elongated in a northwest—southeast direction as 
shown in Plate If. There are no known faults nor igneous intru- 
sions along the boundaries of the block. Bridge (3) points out 
the block-like nature of the area and suggests the possibility of 
