THE CAMBRIAN PERIOD 



489 



tilted, often at high angles. Where strata are vertical, the width 

 of their outcrop on a horizontal surface is about the same as the 

 thickness of the beds (Fig. 364) ; where they are nearly horizontal, 

 as in the left-hand side of Fig. 364, the width of outcrop on a hori- 

 zontal surface is much greater. 



It is not to be inferred, however, that horizontal strata neces- 

 sarily have a wide outcrop. The width of the outcrop is also in- 



Fig. 365. Diagram illustrating the effect of topography on the width of the 



outcrop. 



fluenced by topography, as shown in Fig. 365. Here the horizontal 

 stratum between B and C has about the same thickness as -of Fig. 

 364, but its outcrop is narrow. In general, the width of outcrop, 

 so far as determined by topography, depends on the angle between 

 the bedding-planes and the surface where the formation outcrops. 

 The width of the outcrop decreases as this angle increases. 



Changes in the Cambrian sediments since their deposition. The 

 sediments of the Cambrian system have undergone more or less 



Fig. 366. This figure represents a section in the Menominee region of north- 

 ern Michigan, and shows the Potsdam sandstone, -Gs, in unconformity 

 with older formations. (Van Hise, U. S. Geol. Surv.) 



Fig. 367. Section showing the relation of the Cambrian in the Appalachian 

 Mountains. The strata are both folded and faulted. -G, Cambrian; 

 O, Ordovician; S, Silurian. Length of section, 13 miles. (Hayes, 

 Cleveland (Tenn.) folio, U. S. Geol. Surv. Ordovician and Silurian not 

 separated in the original.) 



change since their deposition. In most regions, the gravels, sands, 

 and muds have been compacted and cemented into conglomerates, 

 sandstones, and shales respectively. In some places, the cemen- 

 tation of the sandstone has gone so far as to convert it into quartzite. 



