ANNIVEESAET ADDRESS OF TB.E PRESIDENT. 69 



one section is concerned, might be either needles, like hornblende, 

 or transverse sections of laminsB, like flakes of mica. The greater 

 part of these lie nearly in the plane of cleavage; but some are 

 inclined to it at a more or less considerable angle. Amongst 

 these flakes or fibres occur objects which look like extremely fine 

 black hairs, their thickness being often less than g-oTooo ^\ ^n 

 inch, which, however, so far as this appearance is concerned might 

 be transverse sections of thin plates. A section cut m^ the line ot 

 strike differs only in showing that the needles or lammse lie less 

 completely in the plane of cleavage. Of course, if the objects 

 which appear to be fibres were really needles, like^ those ot horn- 

 blende, we should also see them of similar form m a section cut 

 in the plane of cleavage, and they would depolarize light as 

 strongly as before; whereas, if they were laming of mica they 

 would show no well-marked outline, because the flakes would lie 

 one over the other, and they would scarcely depolarize, because the 

 light would pass through them in the direction along which there 

 is little or no double refraction. Now, when we examine a section 

 of the above-named black slate so cut, we do see, as betore, the 

 small black hairs, which therefore must really be needles ; but at 

 the same time we' clearly see that the colourless mam constituent 

 of the rock must be in the form of flat plates, as truly lammar as 

 mica, since no more of apparently needle-shaped form are visible 

 than correspond to the transverse sections of those larnmse which 

 are inclined at a more or less considerable angle to the plane ot 

 cleavage. The difference in the appearance is, m fact, analogous 

 to that seen in sections of mica-schist cut at right angles and 

 parallel to the foliation. The result of this structure is that a 

 well-cleaved slate of this type depolarizes like a uniaxial crystal 

 having the principal axis perpendicular to the cleavage. ^ 



It IS quite possible that some much more modern deposits m 

 other parts of the world may have the same structure as these 

 slates ; but I have not met with any in our own country more re- 

 cent than the Devonian. It is, however, not mere y a question ot 

 age since some of the strata of our Silurian and older rocks have a 

 constitution similar to that of modern clays. Whatever may be 

 the true nature of this colourless laminar mineral, its optical cha- 

 racters closely correspond to those of mica, and it has afar stronger 

 depolarizing-power than chlorite. lam inclined to beheve that 

 the black hair-like crystals are magnetite. For a long time I 

 thought it probable that the micaceous mineral had been formed m 

 situ bv an alteration of partiaUy decomposed felspar, so that the 

 rock might be looked upon as analogous to the pseudomorphs ot 

 chlorite or talc already described. Such a crystallization of mica 

 has most unquestionably occurred in some of those fine-grained 

 slaty-looking rocks which are a connectmg-lmk between slates and 

 schists ; but the resulting structure is very different from what I 

 look upon as typical of true slates. When thus formed m situ, 

 the crystals of mica are not stratified, but lie at all possible azi- 

 muths, and, moreover, have collected about special centres ; so that 



