HARDWICKE'S SCIENCE-GOSSIP. 



245 



the bed n below are similar to these running through 

 a. What is the meaning of the want of parallelism 

 of these planes of cleavage ? Now the beds a and n 

 consist of hard and solid clay-slate, while the other 

 beds are soft in comparison — it is in fact an interest- 

 ing case of the refraction of planes of cleavage. Just 

 as a ray of light is bent when passing from a denser 

 to a less dense medium, so were these planes bent 

 when passing from the harder rocks a and n to the 

 less hard ones b\ c — in. The clevagae planes in b, 

 c — in make an angle of only 53° with the horizon, 

 in other words the former — the result of the great 

 mechanical pressure on the strata— when passing 

 from a \.o c were bent or refracted as much as 23°. 

 There is no doubt about this, foi the chief planes of 

 cleavage are easily traceable from a through c, e, &c- 

 Again, in parts of a and ;/, coloured bands or 



Veivy 



Vein 



and dip are not the same. The strike of course of 

 the lodes is about at right angles to that of these veins 

 and the dip is much higher, being from about 60'' 

 to 75°. Among the analogies we note the general 

 parallelism of the veins — the occasional presence of 

 cross or ' counter ' veins — the diversity in richness of 

 the metalliferous portion of the veins, the latter being 

 sometimes absent altogether, while the vein still exists 

 — the crystalline character of the filling, and che general 

 turb-like structure — this latter quality is not nearly 

 so apparent in the lodes, for the nature of the filling 

 in them is often of an extremely complicated character 



MwaVI""" 



Fig. 143- 



*' water-lines " are seen, which denote strata of 

 different density to a and ;/, and the lines of cleavage 

 are bent by these bands as seen in fig. 145. The rocks 

 a and ;;, which are really the " posts " of this quarry 

 are converted into slabs two or three feet thick, while 

 thfe other beds are made into slates | inch thick, or 

 even 5 inch, but these slates are very irregular, often 

 thinning away to a very sharp edge at the sides, and 

 are moreover heavy and discoloured. They are not 

 to be compared to the red and blue slates of North 

 Wales, although they have been used locally to a 

 small extent. They show no traces of iron-pyrites, 

 hence, notwithstanding their clumsiness, they would 

 probably wear well. 



Finally let us see what analogies and what discre- 

 pancies exist between the small white quartz veins of 

 our quarry, and the lodes of the district. The strike 



Fig. 144. 



Fig. 145. 



— the variability of the thickness, strike, and dip of 

 the veins — the presence of vughs, and the marked 

 absence of veins where the cleavage is highly deve- 

 loped. Miners know well that where the country 

 rock becomes fissile, the lode is not only often faulted 

 but is considerably reduced in size and richness ; 

 sometimes it is continued through the fissile bands 

 only as a few insignificant strings, or, should the 

 fissile rock be of very great thickness, the lode even 

 "dies away," entirely, the fissure becoming in fact 

 untraceable, if not quite obliterated. The decompo- 

 sition of the larger veins near the surface is also 

 comparable to what takes place in true lodes. Large 

 lodes near the surface usually become disintegrated. 



