POPULAE EXPLANATION. 19 



accomplished is gne of general illustration, it is thought that 

 the present form of the model will be qnite sufficient. It will 

 be observed, therefore, that the sheets composing it are not of 

 equal value, some representing whole systems, so far as they 

 occur in Iowa, while others represent formations only. The 

 title imprinted on each sheet will still further explain their 

 relative representative value. For further illustration of the 

 same subject, the reader is referred to the section further on, 

 under the head of "'Classification of Iowa Hocks." Lest 

 some should fail to gather a clear idea of the intent of the 

 model, the following detailed explanation is given: 



The first sheet, the complete one, represents the whole lower 

 Silurian system with its six formations mentioned in the table 

 further on. The whole six of these formations, it will be seen, 

 occupy only a small portion of the surface of the State, coming 

 to the surface only in its northeastern corner ; but they may 

 with strict propriety be regarded as extending under all the 

 other formations beneath the whole State, at a gradually 

 increasing depth to the southwestward, as that sheet extends 

 beneath all the other sheets in the model. Next comes the 

 sheet that represents all there is in Iowa of the Upper Silu- 

 rian System, which, by the way, is but a single formation 

 here, although the system contains many other formations 

 elsewhere. This single formation is the Niagara limestone 

 and it rests directly upon the uppermost formation of the 

 Lower Silurian System. The Upper Silurian, as will readily 

 be seen, does not extend so far to the northeastward as the 

 Lower Silurian does, but is bounded by it in that direction 

 on one side and on the opposite side by the Devonian. This 

 refers only to the surface it occupies, for it doubtless extends 

 beneath all the other formations of later age in the State, 

 which are represented as resting upon it by the following 

 sheets in the model. The Devonian System in Iowa, like the 

 Upper Silurian System here also, is represented by only a 

 single formation, which is known as the Hamilton; yet the 

 system elsewhere contains several others as will be seen by 

 reference to the general section on a previous page. 



