626 MR, H. H. THOMAS ON THE MINERALOGICAL [ Novy. 1g02, 
In some cases inclusions are numerous, consisting mostly of quartz, 
and of opaque black particles of doubtful nature; other fragments 
of staurolite occur which are quite free from inclusions of any kind. 
This is one of the most abundant of the heavy minerals in the 
Pebble-Bed, forming fully 20 per cent. of the heavy residue in the 
Budleigh-Salterton area; but it gradually decreases in quantity 
northward, till at Fitzhead in Somerset the percentage is relatively 
small. The greatest percentage was noted from the sand of Berry 
Head (footnote, p. 622). This mineral is mainly confined to the 
Pebble-Bed, although a few grains do occur both above and below 
that horizon.’ 
The Micas. 
White mica is fairly abundant, and occurs in colourless flakes, 
usually about 0°45 to 0°50 millimetre across. A negative bisectrix 
emerges normally, but the angle between the optic axes is variable. 
Much of the mica gives undulose extinction. Inclusions are common, 
and consist of zircon in well-shaped crystals, needles and knee- 
shaped twins of rutile, and fine, colourless, curved and branched 
needles of obscure nature (Newton Poppleford). 
Blue and brown pleochroic needles of tourmaline are also 
common, which lie in intersecting rows and radiating groups. Pleo- 
chroic halos may be noticed rarely round the more minute inclusions, 
Brown mica is not very abundant, but its presence has been 
proved in many of the sands by the flakes of gelatinous silica, 
left after treatment with acids. Where the mineral has escaped 
decomposition, it is of a pale-brown colour with intense pleochroic 
halos round inclusions. 
The micas, so far as one can judge, seem less abundant in the 
Pebble-Bed than in the beds above and below it. 
Felspars.—The majority of the felspar is of the ordinary potash 
variety orthoclase; some microcline, however, occurs, easily 
recognized by its characteristic cross-hatching. Decomposition has 
in many cases gone on to such an extent that the grains have lost 
most of their original characters. Even the freshest grain is 
usually much clouded by decomposition-products. The orthoclase 
sometimes shows the characteristic Carlsbad twinning, but no 
undoubted plagioclase has been met with. | 
Felspar is fairly abundant in the Pebble-Bed sands, reaching, as 
far as one can estimate it, about 6 per cent. of the total mass. 
Sphene occurs in rounded, irregular, feebly transparent, brown 
grains, reaching 0°20 millimetre in diameter, which are traversed by 
rough cracks. The pleochroism is poor. This mineral is not very 
common, but is fairly constant in its distribution ; it occurs also in 
the Upper Bunter and Keuper Marls. 
Grains have been noticed, enclosed in a highly refracting crypto- 
crystalline mass which exhibits a rudely radiate structure. This 
enclosing material is probably a decomposition-product. | 
1 It may be recollected that staurolite has been found in the dune-sands of 
Holland. 
