98 



This is detected by the inclination of the numerous thin layers or 

 ribbons, of a different colour from the rest of the rock, marking 

 the sedimentary structure and the true planes of deposition. The 

 dip of the planes denoting the cleavage is towards the south-south- 

 east, at an average angle of 50°, except where it is affected 

 in the Pennsylvania quarry by a small fault, which, traversing 

 a part of the slate, not only causes a local deviation in the dip of 

 the stratum, but an alteration in that of the cleavage also. 



Several favourable circumstances of structure and position, 

 must combine to adapt any portion of the formation to being 

 quarried for roofing slate with success. 



The rock must be of a fine uniform and compact grain, as 

 free as possible from all crushes or contortions of the stratum, 

 cleaving with facility into thin plates in one direction, and 

 breaking with difficulty in every other. It should be exempt, 

 moreover, from sulphuret of iron, which is often found finely 

 disseminated in the coloured seams or ribbons, and which upon 

 exposure to moisture, soon causes a rapid disintegration of that 

 portion of the mass. The quarry should be situated, if possible, 

 where a small rivulet of water may be conducted over the rock, 

 to preserve it in a moist state, in order to render it more easily 

 and evenly cleaved. 



In splitting and trimming the slates, care is taken to reject the 

 coloured ribbons, lest, in course of lime, they should undei'go 

 decomposition. 



It is somewhat curious, that while the belt of pure slate 

 between any two of these ribbons is almost perfectly uniform in 

 texture and quality, there often prevails a sensible difference in the 

 respects between two adjacent belts, though only separated by a 

 ribbon of a slightly different colour, less than an inch in thickness. 

 It merely marks a difference in the composition of the sediment, 

 before and after that which formed the more heterogeneous 

 ribbons. 



Inferring from the highly cleavable condition and firm grain 

 of much of the slate of the belt at the foot of the Blue Mountain, 

 for many miles east of the Delaware, it would seem not im- 

 probable, that a minute examination of the stratum for roof- 

 ing slate, might be rewarded with success at more points than 

 one. 



