DISTRIBUTION OF PORPHYRY BODIES. 207 



Limestone. As it forms the surface rock over a great part of the area, and 

 has hence been subjected to considerable erosion, it is impossible to deter- 

 mine its maximum thickness. Its original extent to the southwest is also 

 completely unknown, since it, together v/ith the inclosing sedimentary beds, 

 has been entirely eroded off from this portion of the area. Along a certain 

 zone, moreover, it occurs below as well as above the Blue Limestone. This 

 lower body is connected with the upper or main sheet along a line running 

 diagonally through the south edge of Fryer Hill, in a southeast direction, 

 toward upper Long and Derry Hill and West Sheridan, to the northeast of 

 which there are two sheets of porphyry, and to the southwest only one. 

 On this line, which is rather a zone than a line and can, in the nature of 

 things, be only approximately determined, it is found that there is a cross- 

 cutting sheet of porphyry connecting the two sheets to the northeast with 

 the one sheet to the southwest, and the Blue Limestone is in consequence 

 found to be split into wedge-shaped and partially-overlapping bodies. The 

 greatest development of White Porphyry appears to be a little southwest of 

 this zone of cross-cutting, on a line passing through Carbonate, L-on, Printer 

 Boy, and Long and Derry Hills, where it attains in places a possible thick- 

 ness of 1,000 feet. To the northeast of this zone both sheets thin out rap- 

 idly, the lower one before reaching a line running through the forks of 

 Little Evans and along the general course of South Evans gulch, and the 

 upper one at a little distance beyond this line. Along a line running N. 

 50° W. from the saddle east of Ball Mountain to the East Arkansas Valley, 

 at the foot of Canterbury Hill, this upper sheet is entirely wanting for shoi t 

 distances, coming in again, however, northeast of that line. 



Besides these two main sheets there is a very considerable development 

 of White Porphyry along a southeast zone, passing just east of the crest of 

 Ball Mountain, where it occurs in the White Limestone and extends down 

 to the contact of the Archean. It is a significant fact that in this zone it 

 has been proved in two instances to be cutting up through the Archean, in 

 the one case in South Evans gulch, near its mouth, and again on the north 

 side of Iowa amphitheater. 



Gray Porphyry. — Next to the White Porphyry the most important body is 

 the main sheet of Gray Porphyry, which, northeast of the Fryer Hill-Sher- 



