The Pre-Cambrian Structure of Missouri 145 
area about seven miles wide by eleven miles long which has al- 
ready been referred to in connection with the Hogan fault. 
From the above description of the St. Francois Mountains 
block, the characteristics as listed below may be used in recog- 
nizing pre-Cambrian blocks in other parts of the state. 
1. Presence of pre-Cambrian rocks. 
2. Presence of granite intrusions along the supposed 
boundaries of the blocks. 
3. Faults in the Paleozoic rocks along the boundaries. 
4. Straight courses of streams for a considerable dis- 
tance along the boundaries. 
5. Presence of pre-Cambrian and post-Cambrian bas- 
ic dikes along the boundaries. 
6. Northeast or northwest trend of structural and 
physiographic features along the supposed 
boundaries of the block. 
7. Northwest trending faults cutting through the 
block. 
8. Projections and indentations whose sides are 
parallel to the sides of the block. 
9. Mineralization around boundaries of the block. 
Avon block. Northeast of Farmington, in Ste. Gene- 
vieve County, there is an area of Lamotte sandstone surrounded 
by the Bonneterre formation. Weller and St. Clair have called 
this the Farmington anticline (49), but it is here named the 
Avon block from the town of Avon on the southern extremity 
of the uplifted area. 
The boundaries of the area are shown on Plate VII which 
was adapted from the geologic map of Ste. Genevieve County. 
Tarr and Keller (44) describe a basic dike of post-Devonian 
age which occurs along the southeastern border of the Avon 
block and mention the occurrence of other similar dikes in that 
vicinity, Singewald and Milton(40) describe the occurence 
and discuss the petrology and related mineralization of an al- 
noite dike intruded into the Bonneterre formation near Avon. 
More recently Rust (36) has increased the number of known 
