W. M. Hutchings — Sediments from LaJces. 301 



"volcanics," though that mineral is rarely recorded as occurring 

 in the rocks in question. 



Also there is no practical diiference to be observed in the nature 

 of the deposits in different lakes which receive the waters from 

 areas of similar general nature. Bassenthwaite, however, which 

 receives the waste of an area of sedimentary rocks, with an almost 

 total exclusion of " volcanics," has a deposit very different in nature 

 from those of the other lakes. 



Taking the lakes which receive the drainage of volcanic areas 

 pure and simple, the minerals identified are : — 



Quartz. 



Felspars. 



Garnet. 



Eliombic Pyroxene (very scarce and only in Wastwater). 



Hornblendes. 



Musco^dte. 



Biotite. 



Epidote. 



Chlorite. 



Calcite. 



Zircon. 



Tourmaline. 1 • i . • j? . ^ 



■n x-i > in clastic iragments only. 



Hematite. 



Magnetite. 



Amongst the felspars the varieties identified are orthoclase, albite, 

 labradorite, anorthite, and a few bits of microcline.' 



^ The felspar-fragments in these deposits are not -n-ell suited for identification of 

 varieties, as they are to a great extent quite irregular, ragged bits, few of them 

 showing any definite boundaries or cleavages with which to correlate extinctions and 

 optic figures. The larger bits (xio inch) are mainly more or less turbid, like most 

 of the felspars in the rocks from which they are derived ; but among the smaller 

 grains of the more finely-sifted materials there are plenty of clear fresh bits to be 

 found. 



It may be remarked, in this connection, that even with very imperfect material, 

 both in a fragmental condition and in rock-sections, a good deal more can be done in 

 the way of identification than is often supposed, by making use of Schuster's method 

 of extinctions, in conjunction with a careful application of the figures in convergent 

 polarized light. It is often not necessary to have cleavage -flakes sufiiciently good 

 to enable the siffn as well as the angle of extinction to be determined. Thus, suppose 

 we have a very imperfect flake, or an irregular bit in a sUde, showing, however, one 

 good cleavage-crack or boundary. If we find the extinction is 18° — 20°, measured 

 against this line, and if we have at the same time an undoubted emergence of a 

 positive bisectrix not quite central in the field, then we know that we have albite. 

 A similar fragment or bit in a section, giving an extinction of about 36°, with a 

 sharp and distinct axial bar weU within the field, but near its edge, will be anorthite. 

 Other determinations may be made in this way on very unpromising material, 

 provided the optic figures are understood and are carefully used as a guide, and 

 no attempt is made to draw inferences except when these figures can be determined 

 beyond question. 



This is not an appropriate place in which to go more fully into this subject, which 

 can be easily followed out by studying the details given in Eosenbusch, or in Teall's 

 "British Petrography" or other works, as to Schuster's method, and the optic 

 figures on the M flakes of the different felspars. 



To obtain good residts it is necessary, as I pointed out in previous remarks on the 

 determination of felspar (Geol. Mag. January, 1894, p. 41), to have efiicient optical 

 apparatus. In very thin flakes, or in bits in thin sections, owing to the very low 

 polarization-tints, it is often difficult to determine the extinctions with sufficient 



