272 W. M. Hatchings— The Origin of some Slates. 



presence of more or less numerous extremely small flakes of a 

 micaceous mineral. In films a little thicker still, we usually soon 

 reach a point where the action of the granular matter is obscured by 

 the depolarization of this mica, much of which, when mounted in 

 balsam, cannot be made out at all in ordinary light. 



In most of the fireclays I have examined, the mica and other 

 substances (quite apart from actual sand and coarser flakes of mica, 

 etc.) much exceed the "kaoline." The plasticity of such clays seems, 

 as has been suggested by some writers, to depend largely on the 

 fineness of the particles ; also probably on their physical nature 

 connected with the hydration of the mica, because of all these claj's 

 it is true, as it is of more kaolinitic clays, that as soon as they are 

 dehydrated, they are no longer plastic. 



The universal presence of a notable amount of titanic acid in 

 fireclays was announced by Riley in a paper published in 1862 

 (Journ. Chem. Soc. vol. xv.), in which many analyses are given, and 

 where those who are not already aware of it may get full information 

 as to the separation of titanic acid, and may see also how it comes 

 that it is so often left undetermined in analyses of silicates. 



It is now known, of course, that this titanic acid so universally 

 diffused in clays shows itself to the microscope as crystallized rutile, 

 for the most part. 



I think it is now largely considered that in the more advanced 

 shales, and in slates, we have the result not simply of consolidation 

 of original deposits, but also of considerable chemical changes result- 

 ing in the formation of new minerals ; and also that these changes 

 sometimes partly commenced to take place at comparatively early 

 periods after the deposits were formed. 



The object of these notes has been to confirm this in some cases 

 and also to endeavour to trace out, as far as may be, what is the 

 course of such changes in a sediment of known origin and composition. 



The development of so much rutile in the paste of these shales and 

 clays is of interest, because none of the original minerals of the 

 deposits contained it as such. 



In the present state of our knowledge of the composition of these 

 minerals we must be led, I imagine, to the conclusion that the 

 biotite has been the source of this rutile. Biotite has often been 

 proved by analysis to contain titanic acid in combination up to 

 between three and four per cent. So far as I am aware, neither 

 orthoclase nor muscovite has ever been shown to contain any of it. 

 Minute dark hair-like inclosures in quartz of granites are some- 

 times spoken of as rutile, but apparently without definite proof; at 

 other times they are called " schorl," also without proof, while some- 

 times they are spoken of first as one and then as the other, as by 

 Mr. J. A. Phillips in his paper on grits and sandstones (Q.J.Gr.S. 

 vol. xxxvii. 1881) — often on the same page. Granting that they are 

 rutile, as is very probable, the amount of that mineral set free by 

 the wear of quartz, and so introduced into a sediment, would be very 

 minute ; and the quartz of many granites is free from these 

 inclosures. 



