BY W. N. BENSON. 



561 



lowest beds made visible by the fault are the clayshales and 

 chei-ts, immediately underlying the limestone, which is obviously 

 of the same character as the Moore Creek limestone, is silicified 

 in the same way, contains precisely similar fossils, and is asso- 

 ciated "with similar rocks. The silicification of the limestone is 

 very irregularly distributed ; portions of some fossils may be 

 replaced by silica, a sort of beekite, while the remainder may be 

 pure calcite. Again, long or slxjrt, irregular, siliceous bands occur 

 quite apart from the fossils, or, again, the fossils may be found 

 to have merely an outer skin of silicification. The limestones on 

 Spring Creek are divided up into small masses, partly, no doubt, 

 by the plexus of faults (sometimes marked by fault-breccias), 

 but also owing to the fact that they were originally formed as 

 small lenticles, or masses which interdigitated with the cla}^- 

 stones. This is clearly seen in a small cutting below the Cor- 

 poration's stone crushing mill (see Text-fig. 2). These claystones 



Fig 2. — Lenticular coral-limestone and radiolaiiaii claxslone of the Mooie 

 Creek horizon, exposed in road-cutling by the rock crushing plant 

 on Spring Creek, Taniwor th Common. 



are of the normal siliceous type; they are not calcareous mud- 

 stones, such as one might expect if the coral-reef, that is now 

 limestone, had risen to the surface of the sea, and had come 

 under the shattering influence of the waves. In this way, the 

 Tamwortli and Moore Creek limestones differ from the Devonian 

 limestones of Ohio, that were described by Grabau, and from 

 other limestones of a like character(21). 



Neither the Moore Creek nor the Tam worth limestone-occur- 

 rences are closely associated with any igneous rock, though a 

 short distance above the limestone at Tamworth are a few thin 

 bands of pyroclastic rock, and some very narrow layers of fel- 

 spathic tuff, which show the clearest evidence of their intrusive 



