302 W. M. HutcJiings — Sediments from Lahes. 



The hornblende is partly green and fibrous, such as results from 

 the alteration of augite ; but a good many bits of the deep-brown 

 compact variety occur, more probably of original volcanic origin. 



The muscovite is of two kinds. In the finer portion of the 

 materials the mica is abundant as small, usually greenish flakes of 

 sei'icitic nature, such as one sees plentifully in the altered felspars, 

 and in the mass, of the lavas and ashes of the district. In the larger- 

 grained portions of the deposits examined there are larger flakes of 

 muscovite than are seen in these rocks. 



The biotite is all of this larger kind. It is to be recollected that 

 the deposits contain the waste not only from the I'ocks of the district 

 in situ, but also from the materials of the surface-drift, and it is to 

 this latter that we must probably refer some of the minerals found, 

 as notably the microcline and biotite and some of the larger flakes 

 of muscovite, though in the case of Wastwater these latter minerals 

 may have come from granitic rocks in situ. 



Taking Bassenthwaite as representing the deposits from an area 

 of slate-rocks, we still find closely the same qualitative list of minerals, 

 but a totally different quantitative arrangement of them. Thus, 

 felspar is scarce, and among it plagioclase is rare. Chlorite is also 

 scarce, in contrast to its great abundance in the deposits from the 

 " volcanics." The great bulk of the material consists of flakelets 

 of slate, showing quartz and mica, and crowded with minute rutile- 

 needles. The finest portions consist mainly of the minute scales of 

 mica from these slates, with the rutile-needles and with a good 

 many of the small anatase crystals characteristic of clays and shales. 



Tourmaline is present, not only as clastic grains as in the other 

 lakes, but also as the small green hemihedral ci'ystals characteristic 

 of the slates, etc. In the deposits from the " volcanics " the quartz, 

 like the other minerals, is all present as angular fragments or flake- 

 like chips ; the distance it has traversed was too short to allow of 

 any notable amount of rounding of the grains. The quartz in 

 Bassenthwaite is largely in a more smooth and rounded form ; it 

 represents the grains which were already well worn when they 

 were deposited as part of the materials now forming the Skiddaw 

 slates, etc. 



In Derwentwater we have a lake giving a mixed deposit, in which 

 both " volcanics " and " sedimentaries " take part. The former, 

 however, much predominate in an average sample for the entire 

 lake, and the latter are only in evidence by a very few minute slate- 

 fragments, by rutile-needles and by small crystals of tourmaline. 



accuracy. For this purpose a Bertrand quartz-eyepiece (with four quartz segments) 

 will be found of the greatest service. 



The discrimination of the felspars is, of course, of the highest importance in all 

 petrological work, and especially so in the study of metamorpliic rocks, so that too 

 much stress cannot be laid upon the desirability of carrying it out in all cases as 

 fully and as definitely as the nature of the material will allow. It appears to be 

 sometimes left undone, or very vague, where there is no necessity to so leave it ; 

 or it is even left undecided whether a mineral is quartz or felspar, — an uncertainty 

 which is seldom necessary where the mineral or minerals occur in any reasonable 

 quantity in a sKde. 



