42 



is shown in the fact that the clay seams show no bending or 

 btickHng such as would result if much compression of the coal 

 had occurred after the horsebacks were completely formed. 

 Where the lower edge of the coal rests tipon the clay filling 

 of the horseback, the laminae are not curved upward adjacent 

 to the fissure as they would be if the coal at a distance from 

 the fissure had been compressed or settled downward since the 

 horseback was formed. That a degree of consolidation of the 

 coal sufficient to permit of jointing had occurred prior to the 

 formation of the horsebacks is shown by the fact that in some 

 places the clay from the fissures has spread into the joints of 

 the coal adjacent to the horseback. 



Campbell* suggests that joints are developed early in the 

 process of coal formation, and that the carbonization of the 

 coal, beyond the lignite condition, depends upon the presence 

 of joints and cleavage planes along which the gases cotild find 

 a way of escape. If this is the case, there would be a coii 

 siderable amount of compression and contraction of the coal 

 seam after the joints, were formed, before the vegetal mass 

 reached the condition of bituminous coal. 



It is assumed that as the mass of vegetal material, under 

 the weight of overlying sediments, was slowly transformed into 

 coal, there would be somewhat unequal contraction in differ- 

 ent parts of the seam, owing to the lack of homogeneity of the 

 vegetal materials making up the coal beds ; and that the con- 

 traction of the coal materials continued long after a high de- 

 gree of consolidation of the coal had taken place. So long as 

 the materials possessed some degree of mobility the unequal 

 shrinking in the different parts of the coal seam would be 

 equalized by the movement of some of the mass towards the 

 points of least pressure. When the consolidation reached a 

 certain point such adjustment would be no longer possible. 

 After this, the continued uneciual shrinking of the vegetal 

 mass would cause unequal strains in the roof of the coal under 

 its load of superposed sediments. 



If the roof of the coal seam was a soapstone, or somewhat 



'Campbell: Economic Geology, Vol. I, No. 1, p. 30. 



