318 BRACHTOPODA OF THE 



sandstone and quartzite boulders alone that we have to deal. These, as observed, by 

 Mr. Carter, constitute the great bulk of the pebble-bed of the locality. Their position 



each parallelly overlying each other, and also dipping steeply towards the east, viz. first (beginning from the 

 west) a group of Red Clay strata ; second, a thick stratum of Pebbles, which we shall henceforth call the 

 * Old Beach ' (as their form and position bear evidence of its marine accumulation as much as does that of 

 the accumulation now going on here, which, in contradiction, will be called the 'New Beach'), to which 

 we shall now give our chief attention ; and third, Red Sand strata. 



" Beginning again from the west, the stratum of Pebbles may be seen in the cliff to cap Flagstaff Hill, 

 which is about 300 feet above the level of the sea, and slopes downwards for about a mile to disappear 

 under the New Beach, so that the rate of dip of these strata may thus be easily conceived. The pebble-bed 

 is about fifty feet thick and subject, with the other strata, to slight faults here and there in its course ; but 

 it is not our business to deal with them here so much as with the form, position, size, and nature of the 

 pebbles of which this stratum (which is said to extend twelve miles inland) is composed. 



" The form of the pebbles is for the most part elliptical, or subcircular and compressed ; and they lie 

 with their long axes more or less parallel to the stratum ; but they may be more or less globular 

 according to the massiveness or hardness of the rock from which they originally came, In size they vary 

 from a foot in diameter to immeasurable smallness, and in nu mber are inversely proportioned in their 

 largeness. The stratum, too, is often intercalated by small limited beds of sand. 



" We may divide the pebbles of this ' Old Beach ' into Plutonian or Igneous, Neptunian or Aqueous, 

 and Pluto-Neptunian, respectively thus— 



Plutonian. Neptunian. Pluto-Neptunian. 



" 1. Granitoid. 1. Quartzites of different degrees of fineness up to coarse. 



" 2. Porphyroid. 1 . Volcanic breccia. 



" 3. Volcanic. la. Grits or pudding-stone. 



" PLUTONIAN. 



" 1. Granitoid. — These are for the most part composed of a fine crystalline base embedding the same 

 materials in larger portions, viz. hyaline Quartz, Felspar, and Hornblende. Of these three ingredients 

 the crystals of Felspar are most striking, from their opaque pink colour and large size; next in size and 

 regularity of form comes the Hornblende, and lastly the hyaline Quartz. Such pebbles I have found of 

 all sizes, from nearly a foot in diameter downwards. The mineral combination mentioned may be found 

 under different aspects — thus, the pink opaque Feldspar may be transparent, colourless, and glassy, 

 or the Hornblende may be actinolitic, and the Quartz recrystallized in prisms, &c, but however much 

 these changes, partly due to decomposition, as well as the relative form and nature of the ingredients, may 

 give rise to varieties, they chiefly characterise the rock from which these pebbles were derived. Mica 

 I have never seen, unless the glittering particles which equally characterise the Red Sandstone generally, as 

 well as the pebbles of the old beach, is to be regarded as such. Opaque, white quartz pebbles from the 

 granitoid rock are occasionally found charged with pellets of a light yellow, glistening, laminated mineral, 

 which, from its softness and almost inflexibility, enrpyrically 1 should set down as talc, as well as the 

 glistening particles just mentioned. Still an Avanturine pebble is not uncommon. 



" 2. Porphyroid. — Such are pebbles consisting of compact grey or reddish feldspathic base embedding 

 round grains of hyaline quartz, and rhombic crystals of pink feldspar. Like the foregoing, the feldspathic, 

 crystals may have other minerals embedded in them, but the staple composition of the rock is that men- 

 tioned. I never found pebbles of it more than six inches in diameter. Its local name is said to be 'Elvan 

 Stone,' and it is also said that it occurs as dykes in the granitoid rocks of Dartmoor and Cornwall. 



"3. Volcanic. — I would assign this name to dark, black, greenish pebbles whose composition appears 



