Fig. 309. Map showing the outcrops (in black) of Lower and Middle Cambrian 

 formations. The areas shaded by lines represent regions where the formations are 

 believed to exist, though not exposed. The longer the lines, the better the basis 

 for belief in the existence of the beds. The unshaded areas north of Mexico are 

 believed to have been land during the early portion of the Cambrian period. The 

 unshaded area south of the United States represents lack of knowledge. The 

 shading within the area of the ocean is the same as in Fig. 295. The Middle Cam- 

 brian may be somewhat more extensive than the map shows. The area not covered 

 by the early Cambrian formations, and probably a still larger area, was land at the 

 end of the Proterozoic. 



