THE LOWER SILURIAN ROCKS. 551 



that numerous small, often somewhat distant, exposures have to be 

 thrown into place by means of aneroid observations, a not very satis- 

 factory guide. The other difficulty arises from the irregular nature 

 of the upper surface of the formation, which is due to erosion before 

 the deposition of the St. Peters sandstone, and as a result of which 

 the Lower Magnesian varies from 50 to 250 feet in thickness. From 

 this it follows that all determinations of horizon within the Lower 

 Magnesian, made by measuring from the base of the St. Peters sand- 

 stone downwards, are worthless. Varying so greatly as it does in thick- 

 ness, beds immediately underneath the St. Peters may, in fact, be 

 nearer the base than the summit of the formation. 



In the region around Madison, the Lower Magnesian has usually a 

 thickness of about TO to 80 feet, though it may run from 50 to 100. 

 feet, The following outline scheme of the succession of its layers in 

 this region is an abridgement of a more elaborate one, which ac- 

 companied the annual report on the field work of 1874, to which was 

 also attached a large chart of grouped sections. These are not here 

 reproduced, because the work of later seasons proved them to have 

 only a local value. The numbers in the scheme are from below 

 upwards, on account of the uncertainty as to any measurements from 

 above, downwards. The exposures cited are only single instances out 

 of many representing the different layers. 



Ft. In. 

 VII. Concretionary, brownish-yellow layers, which at times are quite sandy, 



and at others contain not more than 3 to 4 per cent of fine aluminous 

 impurities; Veerhusen's quarry, Sec. 25, Westport; Williams' quarry, 

 Sec. 38, Madison. Tliickness about 10 



VI. Heavy, indistinctly bedded layers, which appear to vary much in charac- 

 ter, being sometimes, as at Williams' quarry, Sec. 33, Madison, greyish- 

 white, nearly free from foreign ingredients, minutely crystalline, con- 

 choidal-fracturing and compact, but with small cavities lined with dolo- 

 mite crystals. Interstratified are continuous seams, and rows, of white 

 surfaced nodules of jaspery chert. At other times, as at Veerhusen's 

 quarry, Sec. 25, Westport, these layers have a greenish-tinted buff color, 

 and a fine-granular texture, containing 30 to 40 per cent, of fine quartz 



sand, and little or no chert. Thickness, in all, about 15 



V. Thin, regularly bedded, greenish yellow, fine-granular layers, with very 

 abundant and large dendritic markings; Veerhusen's quarry 10 



IV. Very irregularly bedded, alternatingly heavy and thin, white-and-yel- 

 low- mottled, rough-textured, very cherty, layers; Middleton quarry. ... 20 



III. Thin, irregular, usually somewhat sandy, brownish layers, including oc- 

 casionally beds of 1 to 2 feet in thickness. Near the top a very highly 

 brecciated layer, 1 foot thick, often comes in (quarry near the school 

 house, Middleton); wliilst below, oolitic chert pervades the limestone, and 

 alternations of more and less sandy layers occur, constituting a passage 

 downwards into the Madison sandstone (Middleton and Madison quar- 

 ries). In some cases this gradation is not marked, the transition from a 



