IOWA ACADEMY OF SCIENCES. 45 



The well known higher porphyry mountains may frequently be recognized 

 many miles away; their position, and consequently name, being readily 

 detected, owing to the peculiar or distinct topographic features characteristic 

 of the individual mountains. Famous Pilot Knob and its neighbor just across 

 the valley of Knob creek, Shepherd mountain, are excellent examples of such 

 forms. In the case of the former it is especially so, for, besides standing out a 

 rather sharp, conical mountain, singular in form it is also marked by the deep 

 cut from which iron has been mined for years which extends almost to its sum- 

 mit. Although this mountain is not so high by nearly 300 feet as some 

 others, the distinctive form which it possesses together with the artilicial cut 

 makes its recognition doubly easy. 



In the extreme southeastern portion of the State extending northward 

 from the Arkansas-Missouri line and westward from the Mississippi river 

 lies what is known as the earthquake region. This is now a rather exten- 

 sive territor^^ composed for the most part of lowlands, swamps and marshes. 

 The lowlands commonly rising not many feet above the "Father of Waters" 

 on the east are of Tertiary and Qnatern-ai'y age. Grading seemingly some- 

 what gradually on the west on account of the contact with the low or l)ot- 

 tom land naturally approaching the waters of the Black river, and quite 

 abruptly on the north, the topography of the swamp region stands in marked 

 contrast with the rough topography of the Archa3an and Cambrian hills; 

 the first with far sepai*ate contours and sluggish streams, the last with a 

 magnificent drainage, high hills and narrow valleys. 



As has been said, each formation in distinct area, whether it be porphyry, 

 granite, limestone, sandstone or a combination of two or more of these will 

 have its own special type of topography, each peculiar in' itself as well as 

 when compared with others. So by means of the topographic maps one 

 can ordinarly discern the formations represented thereon. 



Whilst in territories of limestones and sandstones the number of streams 

 possessed by each is nearly the same, such may be said of granite and 

 porphyry fields; but the number of larger streams and stream-ways in the 

 Cambrian greatly exceeds the number in the Archasan. The streams in 

 the former are more tortuous, the channels considerably wider and the flow 

 less rapid. This is all largely due to the great difference in the texture of 

 the rocks of the two geological formations, the compai'ative softness of the 

 sedimentaries augmenting erosion. What is but a dull drainage line in the 

 crystallines becomes, in a corresponding period, a well marked ravine in 

 the sedimentaries. 



The regularity with which the Archa!an streams have been and are 

 being formed depends primarily upon the form of the upland. If it con- 

 sists of hilltop after hilltop the streams or gullies will be more common and 

 more strongly marked than if the summit is not pointed, but is a narrow 

 or wide plane of some length. 



The limestone areas in southeastern Missouri are of two kinds; the com- 

 mon is the irregularly broken ridge with a crest having about the same level, 

 from which extends more or less successively, often for a considerable dis- 

 tance, points or spurs of various lengths. Nearly the whole surface is cov- 

 ered with detritus which consists mainly of chert fragments often coated 

 with drusy quartz. These ridges made up for the most part of the Mag- 

 nesian limestones in heavy ledges, are only recognized as bearing such by 



