220 



GEOLOGY. 



TT?i 



Fig. 90. — Map showing the outcrops (in black) of Early and Middle Cambrian forma- 

 tions. 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 knowl- 

 edge. The shading within the area of the ocean is the same as in Fig. 38. 



