IOWA ACADEMY OF SCIENCES. 47 



tioned. These occasionally do resemble in outline the lower pointed por- 

 phyry knolls, though the angle of slope is commonly greater in the latter 

 and the contours less sinuous. 



From the sandstone and limestone uplands great hills of granite or por- 

 phyry not unfrequbutly ascend abruptly to diversitied heights. Often do 

 the sedimentary rocks almost completely conceal the crystalline. In other 

 cases the stratified rock now extend only a short distance up the hill and 

 across the valleys and shallow divides. 



As a rule the porphyry mountains are either pointed at the top or have a 

 long, narrow crest, but occasionally large mountains have summits quite 

 broad, grading into the steeper hillsides; Taum Sauk mountain, perhaps 

 the highest in the region, is a good example of this form. 



The contouring of these mountains is quite plain, yet distinguished in 

 being as a whole different from that of other hills in this region. The angle 

 of slope from base to summit is, of the larger mountains, almost constant, 

 no matter whether this angle be small or great, whether the mountain top 

 be pointed or narrow crested. Pilot Knob may again be taken as an exam- 

 ple to illustrate the former and Buzzard mountain, adjoining the Knob on 

 the north, to illustrate the latter. 



Over the inclines of the steeper porphyry hills great blocks and fragments 

 of the rock have accumulated as they weathered from the body mass. This 

 detritus is often of great thickness and hides the solid rock except in case 

 of almost perpendicular faces. Thus it modities somewhat the otherwise 

 rugged surface. Soily material is here commonly very thin, and vegetation 

 is not abundant and the rocks are of slow decomposition. 



Canons are not unfrequent in their appearance over the Archrean region. 

 They are found of variable lengths in the granite as well as in the porphy- 

 ries. The many water-courses, in seeking an outlet, have cut through 

 great bodies or hills of these excessively hard beds, which as yet confine the 

 water to very narrow channels with more or less expansive precipitous 

 walls, bare and rugged in outline. The beds of the streams are broken and 

 waterfalls abound. 



It has been mentioned incidentally that the topography of a granite field 

 is essentially different from that of the porphyry. The porphyries, almost 

 without exception, have an aphanitic structure, whilst the granites are 

 often extremely coarsely crystalline and are, therefore, subject to much 

 more rapid erosion, erosion on all exposed faces to about the same extent 

 leaving a rounded surface in every case. Of the porphyry, weathering 

 takes place not in decomposition of the surface, but by merely a sepa- 

 ration of the stone into blocks and fragments along joint planes, in this 

 rock always numerous whilst comparatively rare in the granite. Occa- 

 sionally large blocks of granite break away along joint planes and, weather- 

 ing, are transformed into huge boulders, which either remain on the solid 

 rock bed or tumble into the streams at the foot of the mountain. 



Granite mountains are commonly rounded at the tops and often the 

 upper gently rolling surface extends over quite a large territory. They are 

 of less height and may make up the greater part of a mountain whose highest 

 point is of porphyry. The slopes are irregular and broken. To map (using 

 compass, aneroid and level) with accuracy and detail, a field of granite 

 must necessarily be traversed at frequent intervals, perhaps more so than is 



