STRUCTURE 



665 



lu contrast to tlie porous nature of the bedding planes in the Pottsville 

 rocks mentioned above, the contact of the Niagaran limestone with the 

 Maqnoketa shale where the sand is absent appears to be impervious, and 

 for this reason accumulation of oil and gas takes place in each lens inde- 

 pendently. The lower part of the sand on the terrace at the northeast 

 side of the dome is filled with salt water, as is also the portion of the 



One mile 



FiGUKE 4. — Sketch showing Significance of Unconformities and the "spotty" Character 



of Sand in Colmar Area 



dome farther down the dip, but the lens of sand at the top of the dome 

 also has its proportion of salt water, which lies about 70 feet higher than 

 that on the terrace. 



A number of domes equal in magnitude to that at Colmar have been 

 located in the western part of Illinois, but outside of the Colmar area no 



