36 The Ottawa Naturalist. [May 



distance of several feet. Apparently the conditions which allow 

 of the formation of mud-cracks (see fig. 1) also postulate a slight 

 variation in the composition of the limy muds originally de- 

 posited. Thus, a series of alternating layers, which have been 

 successively cracked by dessication, when viewed at right angles 

 to their plane of deposition, show a series of stratified brecciated 

 fragments. It is interesting to note that where quarries have 

 been opened in the Bellefonte section (at both the middle Beek- 

 mantown and Lowville horizons) so as to expose the limestone 

 beds for some distance along both the dip and strike, great mud- 

 cracked areas have been brought to view. The writer has seen 

 a mud-cracked surface on the west wall of the quarries at 

 Tyrone which was at least one-half an acre in area. Only the 

 closest inspection, however, of the section across or at right 

 angles to the dip will show any structure that might lead the 

 stratigrapher to suppose that mud-cracks were present, and in 

 such great abundance. When the filling of the cracks, or rather, 

 the material surrounding the phenoclasts, is of a different colour 

 or texture from that of the phenoclasts themselves, a stratified 

 intraformational breccia often proclaims that its other name 

 is "mud-crack." Thus, in a region such as that characterized 

 by the Appalachian type of folds, where the rocks are usually 

 observed at an angle of between 2 5 and 60 degrees, it is quite 

 natural that mud-cracks, and ripple-marks should be considered 

 rare phenomena, except where exposed in quarries and road-cuts 

 along the strike. The mud-crack zone may have a strati- 

 graphic thickness of only 3 or 4 feet and yet extend along the 

 strike a distance as great as that from Bellefonte to Tyrone (60 

 miles), or even farther. What the total area of such a mud- 

 cracked surface might amount to is difficult to surmise. Owing 

 to the fact that the dip of the limestones at Pleasant Gap, several 

 miles east of Bellefonte, is considerably flatter than the dip of 

 the same beds at the latter place, the writer has been unable to 

 get, as yet, any exact data as to the geographical extent of this 

 phenomenon, but all signs point to its being an exceptionally 

 wide one. 



In connection with this subject it might be well to mention 

 a certain columnar structure observed and described by E. M. 

 Kindle (5) in the Silurian limestone on Temiscouata Lake, in 

 eastern Quebec. The occurrence of columnar structure in lime- 

 stone is unusual, and very like basaltic columnar structure in 

 general, "but the columns are perhaps less regular in the num- 

 ber of faces shown, five to seven being a common number." 



(To be continued). 





