338 i 



In the case of very large rolls the effects above described 

 are accentuated. The coal over the crest of the roll has 

 been forced away to the sides so that the floor rocks come 

 into contact with the roof. The roof also is affected and 

 takes the form of the top of the roll. In extreme cases 

 the crest of the roll is faulted, as in Fig. 6 (from a drawing 

 supplied by Mr. H. L. Garlick, of South Bulli). 



Further evidence of compression is afforded by the 

 phenomena of the basaltic intrusions in the coal seam. 

 Basalt dykes are numerous and, where they intersect the 

 coal seam, they often spread out into extensive sills. In 

 at least one instance in the Mount Keira Mine such a sill 

 has been abruptly truncated at the crest of a " tie roll" 

 the density of the coal being sufficient to prevent further 

 spreading along the plane of the seam. 



According to Mr. Garlick, who has made these rolls the 

 subject of careful study, if the roll is traced downwards it 

 is found to die out gradually. The four foot sandstone is 

 never affected, and the underlying four foot seam shows 

 no trace of disturbance. 



Cause of the Rolls.— The phenomena of compression 

 described above point to the folding of the floor being long 

 subsequent to the date of formation of the coal. This 

 negatives the suggestion that the rolls are due to slip in 

 the unconsolidated muds of the coal swamp as a result of 

 local overloading with river sediment. Further, there is 

 no evidence, which I have seen, of the occurrence of gravel 

 banks competent to produce such au effect. 



That the rolls are not ordinary contraction folds, caused 

 by uniform shrinkage of the earth's crust, is proved by their 

 localization in the floor of the seam. If caused by so far 

 reaching a force as that mentioned they would certainly 

 affect the associated beds also. 



