W. M. Hutchings—The Origin of some Slates. 269 



In addition to the grains of quartz and felspar and the more pro- 

 nounced flakes of inuscovite and variously altered biotite, as above 

 described, the sections show the presence of another sort of com- 

 ponent which lies in among these former, and in some parts more or 

 less surrounds them, after the manner of a sort of ground-mass, or 

 " paste," so to speak. It contains much that is quite indeterminable, — 

 indistinct granular matter with specks and microlites of various 

 sorts, and with all this a great deal of a fine, micaceous, dimly 

 depolarizing material. Jn among this may be seen larger flakes of 

 a mica which differs distinctly, in many ways, from any of the 

 original clastic mineral. 



All this is better studied in sections from a much finer-grained 

 portion of the deposit, intermediate between that described above 

 and the finest clay-bands. In such sections all the appearances 

 above noted are seen, but the " paste " bears a much larger propor- 

 tion to the other constituents. It doubtless represents the finest 

 portion of the original deposit, the finest mud or silt, made up of 

 " kaoline," the minutest powder of felspar and quartz, and the 

 smallest flakes and shreds of mica. Throughout this " paste" (I will 

 use the word here for convenience, though not well suited) immense 

 numbers of minute rutile crystals are seen, the larger ones transpa- 

 rent, many twinned, vividly depolai'izing ; the smaller ones as dark, 

 hair-like needles. Also a good many very thin small flakes of 

 micaceous ilmenite, transparent with the characteristic shade of 

 brown, and many very small perfect crystals of tourmaline. 



There is a considerable amount of the mica l above referred to, 

 evidently secondary, formed in situ. It is possible to trace the dis- 

 tinctly clastic mica, more especially the muscovite, down to very 

 minute fragments indeed (correspondingly small biotite is not seen, 

 as such finely divided parts are soon wholly obliterated by altera- 

 tion). Then comes a point where it is often not possible to form 

 any judgment as to whether what is seen is original or secondary. 

 But there is much where no such doubt is called for. Flakes of 

 various sizes, but all small, are seen, colourless or more usually pale 

 greenish or yellowish. The larger ones blend away at their edges 

 with the surrounding fine material in a manner which none of the 

 original flakes of muscovite of similar size do, and they are all more 

 or less full of the minute rutile crystals, not a trace of which occurs 

 in any of the original muscovite. Here and there, though rarely, 

 flakes may be got which are of sufficient size and thickness to test 

 in convergent light. The optic axial angle is usually approximately 

 the same as that of the original muscovite present, but in some cases 

 it is noticeably less. 



This newly-formed mica is not uniformly diffused as to its larger 

 flakes, but the finer-grained paste in which it lies, and of which it 



1 It should be understood that by the use of the word mica there is no intention 

 to infer that anything sufficiently definite is ascertainable about the substance in 

 question to enable it to be certainly referred to any recognized variety of mica. The 

 word is used only to express the fact that the observable characters of the mineral, 

 including such of its optic properties as can be ascertained, briug it into close 

 resemblance to some of our known micas. 



