78 BASALTIC ROCKS, 
stratified rocks. In one locality the trap is based on a singular frag- 
mentary rock of variable composition ; in some points it is an argil- 
laceous breccia, reminding one of the red argillaceous rock of Derwent- 
Water ; in other parts crystalline limestone, variously coloured and 
holding different crystals, enters largely into the mixture; some por- 
tions have a porphyritic aspect ; many look like a volcanic breccia ; 
a few seeming beds occur of a quartzose compound, almost exactly 
like grauwake, and closely resembling some local beds which cover the 
slate deposit in Ribblesdale. The variation of the thickness of the basalt 
is remarkable and sudden : in Hilton beck four fathoms; in Knock Ore 
gill ten fathoms, in Tynedale twenty fathoms, at Caldron snout in 
Teesdale thirty or forty fathoms. It is a compound of white felspar 
and black pyroxene, the latter generally predominating ; generally fine 
grained, but where its thickness is considerable showing also coarse 
granular parts, like some greenstones of Scotland. Contemporaneous 
veins of greenstone with large curved crystals, (hypersthene ?) pass 
through the general mass near High force and Caldron snout ; strings 
of spar, quartz, and veins of lead ore (Hilton, Dufton, Trout beck, 
Tynedale, and Bavington,) occur. The Whin sill is almost in all 
localities rudely prismatic : the prisms being perpendicular to the 
planes of the including strata, and not regularly cross-jointed. It 
appeared in some instances (Miner’s bridge) that the lower part of the 
rock was finer in the grain than the other parts, more jointed and less 
prismatic, — but the latter circumstance is reversed at the High force. 
Considerable chemical changes are produced by the trap rock on 
the limestone, gritstone, and shale, with which it comes in contact or 
proximity. Not far from the south bank of the Tees, about Unthank 
and Holwick, and in the river at Winch bridge, sandstone and shale 
lying tinder the Whin, are greatly indurated, bleached, and full of 
joints ; near the High force some portions of gritstone appear (in- 
cluded ?) in contact with the Whin, are bleached in a singular manner, 
rendered very brittle and full of cracks. Shale or plate is so much 
altered at the High force in the relations of the joints, that most 
persons mistake a part of the prismatic masses really composed of 
