108 



have been viewed too much in the sense of dry, hard and inert, 

 instead of active and moist masses : an absolute dry and compact 

 rock would be a phenomenon in nature, i. e. an extraordinary 

 exception to the usual natural production. 



It is exceedingly difficult to expel moisture from rocks, and 

 when expelled the difference in weight is found very considerable. 

 Limestones contain a great quantity of water, and are sometimes 

 found at great depths very soft; and often, by means of the 

 weight of the superincumbent beds, force a great proportion of 

 their soft parts into the joints of the bounding beds. Some- 

 times limestone beds and hornblende veins have been thus in- 

 troduced between the planes of sedimentary beds. As this part 

 of the subject has been anticipated by the observations made on 

 the nature of the crystalline rocks, we need not extend them 

 here ; we shall conclude by stating that stratified rocks are not 

 homogeneous inflexible dry and solid masses, on the contrary 

 they are composed of numerous moist and pliable sheets, capable 

 of extension and compression, and easily twisted and bent under 

 pressure. 



We have already alluded to the effects of cleavage, i. e. that 

 all the sedimentary rocks are liable to be cleaved by the primary 

 base* : this is particularly the case with coal, and which greatly 

 facilitates the extraction of the coal-seams. Parallel to the strati- 

 fication of the seam are, first, the partings ; intersecting these, 

 in a more or less vertical position, are the polar cleavages : these 

 last separations are sometimes called backs or slines, or bright 

 heads, from the coals separating at these cleavages with clean and 

 highly polished surfaces, except when, as often happens, the 

 complanatory lustre is covered with a rusty-looking scale, or with 

 the well-known white sparry concretion, consisting for the most 

 part of carbonate of lime, derived from the infiltration of the so- 

 lutions of the bounding beds ; besides these fissures, there are 

 others passing nearly .at right angles to the planes of cleavage, 

 fractured divisions, and appropriately denominated cutters : so 

 that by means of this compound system of natural series of lines, 



* Attempts have been made to distinguish sedimentary beds by the cleavage 

 alone, but this is no guide, because any bed or compact soil even is liable to be 

 cleaved by the primary in favourable localities. In New Grenada the soil is 

 often found cleaved subsequently to the growth of the oldest trees. 



