CORNISH POST-TERTIARY GEOLOGY. 11 



the surface is studded with diallage boulders. Near Cowissack I 

 noticed that the boulders rested on soil of drab and light-brown 

 sandy loam, with small angular pieces of quartz, which was exposed 

 by a pond to the depth of one foot. 



Mr. Budge ' says : " The whole country from Porthoustock to 

 Gwinter, four miles, is strewn with a broad band of scattered 

 boulders of a rock locally called Ironstone. A little to the west of 

 Crousa Down a stratum, commonly termed Marie, of decomposed 

 syenitic rock of the same character as the boulders on the Down, is 

 met with." 



Quartz Gravel in the Parish of Crow an. 



Mr. Tyack - notices the occurrence of quartz pebbles (evidently 

 derived from veins in the Killas) resting on granite, near Polorebo, 

 in the parish of Crowan, at about 400 feet above the sea, and about 

 150 feet lower than the lowest part of the adjacent watershed, 

 which separates the basins of the rivers Cober and Hayle. The 

 pebbles vary from the size of a pumpkin to that of a hazel nut ; 

 they are less rounded than beach pebbles, but more worn than those 

 in the river gravels of the neighbourhood. The pebbles occur in 

 surface soil, and in a subsoil of yellow clay, resulting from decom- 

 position of granite ; the larger pebbles have been found here and 

 there in the clay subsoil, in pits sunk to depths of from two to eight 

 feet. The area covered by the gravel is about 800 yards from north 

 to south, and 500 from east to west. The neighbouring granite hills 

 rise at least 100 feet higher than the site of the gravel, but it occurs at 

 a greater elevation " than any clay slate anywhere near it." In the 

 valley below the gravel patch, and one mile distant from it, similar 

 pebbles have been found mixed with refuse from the stream-tin 

 works. 



Between Trewavas Head and PortMeven. 



South of Tremearne houses, I noticed five feet of drab loamy clay 

 or earthy soil, with small angular slate fragments, and containing 

 near the surface numerous small well-rounded and smoothed flint 

 and quartz pebbles. The pebbles are found in brown earthy top 

 soil, for five chains eastward from a little stream which trickles 



1 Trans. Eoy. Geol. Soc. Corn. vol. vi. p. 95. 



2 Trans. Eoy. Geol. Soc. Corn. vol. ix. part 2, p. 177, etc. 



