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Parr IL. § vi] FRAGMENTAL ROCKS—ORGANIC. 165 
Peperino.—A dark brown earthy or granular tuff found in con- 
siderable quantity among the Alban Hills near Rome, and containing 
abundant crystals of augite, mica, leucite, magnetite; and fragments 
of crystalline limestone, basalt, and leucite-lava. 
Palagonite-Tuff.—A. bedded aggregate of dust and fragments of 
basaltic lava, among which are conspicuous angular pieces and minute 
granules of the pale yellow, green, red, or brown basic glass called 
palagonite. This vitreous substance is intimately related to the 
basalts. It appears to have gathered within volcanic vents and 
to have been emptied: thence, not in streams, but by successive 
aeriform explosions, and to have been subsequently more or less 
altered. The percentage composition of a specimen from the 
typical locality, Palagonia, in the Val di Noto, Sicily, was estimated 
by S. von Waltershausen to be silica, 41:26-; alumina, 8°60; ferric 
oxide, 25°32; lime, 5°59; magnesia, 4°84; potash, 0°54; soda, 1:06 ; 
water, 12°79. This rock is largely developed among the products of 
the Icelandic and Sicilian volcanoes; it occurs also in the Eifel and 
in Nassau. It has recently been found to be one of the characteristic 
features of tuffs of Carboniferous age in Central Scotland? (Fig. 27). 
Schalstein.—Under this name German petrographers have 
placed a variety of rocks which consist of a green, grey, red, or mottled 
_ diabase-tuff impregnated with carbonate of hme and mixed with 
calcareous and argillaceous mud. They are interstratified with the 
Devonian formaticns of Nassau and the Harz, and with the Silurian 
rocks of Bohemia. They sometimes contain fragments of clay-slate, 
and are occasionally fossiliferous:.. They present amygdaloidal and 
porphyritic, as well as: perfectly laminated structures. Probably 
they are in most cases true tuffs, but sometimes they may be forms 
of diabase-lavas, which, like the stratified formations in which they 
lie, have undergone alteration, and in particular have acquired a 
‘more or less distinctly fissile structure.” 
4. Fragmental Rocks of Organic Origin. 
This series includes deposits formed either by the growth and 
decay of organisms 7m situ, or by the transport and subsequent 
accumulation of their remains. These may be conveniently grouped, 
according to the predominant chemical ingredient, into Calcareous, 
Siliceous, Phosphatic, Carbonaceous, and Ferruginous. 
(1.) Calcareous.. 
Besides the calcareous formations above deseribed (p: 111) among 
the stratified crystalline rocks as resulting from the deposition of 
chemical precipitates, a still more important series is derived from 
1 Trans. Roy. Sec. Edin. xxix. p. 514. 
2 On some foliated igneous rocks in the “ Killas” of Cornwall, see J. A. Phillips, 
Q J. Geol. Soc. xxxii. p. 155, xxxiv. p. 471. 
