OBERMITTWEIDA CONGLOMERATE. 27 



third are far more abundant than the second. It is, of course, not 

 very easy to settle where fragments are to end and matrix is to begin ; 

 but we may, I think, say that the quartz constituting the latter 

 occurs in angular or subangular grains, in diameter from about -025" 

 (even these being sometimes compound grains) downwards to less 

 than -001" ; but here it becomes quite impossible to say whether 

 one is dealing with original or secondary granules. The felspar is 

 in similar grains, in good preservation but not common : much of 

 it resembles orthoclase, but plagioclase may be identified ; other 

 grains exhibit the vein-like association which may be seen in micro- 

 cline, perthite, &c. The mica is of two species, but most of it is of a 

 strong olive-brown colour, markedly dichroic, changing from a rich 

 olive-brown to a pale buff tint ; the individual flakes do not gen- 

 erally much exceed -003" in length, and are very commonly about 

 this size, though not seldom less. The flakes are often associated. 

 The other mica is colourless. The constituents exhibit a slight 

 tendency to parallelism; but there are no marked indications of 

 " squeezing," not, indeed, so much as I should have expected from 

 the macroscopic aspect of the blocks. The quartz grains are generally 

 rather clear ; the most frequent enclosures are small films of pale 

 olive-coloured to white mica ; so far as I have observed, fluid-cavities 

 with bubbles are neither large nor numerous. While the fragmental 

 character of these quartz grains cannot be doubted, they do not ex- 

 hibit quite the ordinary appearance of fragments in a sandstone or 

 greywacke. In these the boundaries are sharp and definite, the 

 distinction often being accentuated by interstitial material, which will 

 be quite as accurately defined by designating it " earthy dust," as by 

 any more high-sounding appellation. Of this material, so abundant 

 in many greywackes, the residue of decomposed felspar, comminuted 

 mica, and the like, there is here little to be seen. Where two fragments 

 of clean quartz have been in contact, the grains are united with a 

 rather wavy boundary, as in many pure quartzites ; and in the place 

 of the above residue we have tiny mica-flakes, mostly brownish in 

 colour, imbedded in quartz ; the latter mineral being sometimes con- 

 tinuous with one of the larger grains. I have not been able to dis- 

 tinguish the precise boundary line of the original fragments, but 

 think they have been only slightly increased in size by secondary 

 quartz. 



The way in which the mica is disposed in the rock leads me to- 

 regard it as to a considerable extent an original constituent, but 

 here also a change has taken place. The sections of the flakes have 

 not the ragged outline, especially at the ends, common with deri- 

 vative micas in a clastic rock, but resemble those in an undisturbed 

 gneiss or granite, or in a rock aff'ected by contact-metamorphism, 

 giving abundant rectilinear sections, which are rhomboidal in form 

 (see fig.) ; very minute flakes of colourless mica are sometimes asso- 

 ciated with the other, and also occur in the felspar fragments, 

 which, however, are on the whole in good preservation. One or two 

 quartz grains contain hexagonal prisms, which may be apatite, and 

 there are probably one or two fragmental garnets. 



The pebbles and other larger fragments in the Obermittweida 



