216 PROFESSOR FORBES ON THE GEOLOGY 
referred to. The most perfect green rock is found at (10) to the north-west of 
the summit, where it occurs chiefly in large detached blocks and nearly in con- 
tact with the porphyry (11) without much apparent intermixture. Here, then, 
the contrast is sharpest. On the opposite or south-east side of the hill-top, which 
is steep, the transition is imperceptible. The top is a yellowish-grey slaty rock 
passing into red slaty felspar. ; 
Should these views be confirmed by other observers, we have here an interesting 
and accessible example of that mysterious operation of metamorphic action which 
is now so much insisted on by geologists, but of whose exact physical and chemi- 
cal nature we know so little. 
The col uniting the two higher Eildons is composed of a hard red porphyry (15) 
having rude cleavage planes running nearly east and west, and dipping to north. 
The north-eastern summit of the Eildons I examined carefully without tracing in 
any point its junction with other rocks ; it is composed of a remarkably uniform 
claystone porphyry. I did not perceive any imbedded fragments of greywacke 
in this part of the hill such as seem to be referred to by Mr Mine. 
The south-western hill of the three-headed Eildon group includes, as is well 
known, a felspar porphyry in rather perfect columns. I observed no other pecu- 
liarity. As already observed, the whole south-eastern base of the group is so 
hopelessly covered by soil and drift that the examination of the subjacent rocks 
is impossible. 
LIST OF SPECIMENS, 
(now in the Museum of the Royal Society.) 
1.a, Hardened trap tufa, back of Quarry Hill. 
1.b. Trap tufa, Quarry Hill. 
Tex” Do? do., 
2. Altered greywacke, Matty’s Burn, below Dingleton Mains. 
P 
Porphyry, including quartz, above Dingleton Mains. 
Greywacke sandstone on Eildon, below co/, west side. 
Hardened greywacke on Hildon, near co/, west side. 
Felspar porphyry, foot of cone of centre Eildon. 
Felspar rock with green patches, centre Hildon. 
Do. do. 
10. Altered greywacke, north-west of centre Hildon top. 
11. Porphyry, near greywacke slate, No. 10. 
12. Altered greywacke slate, shot with felspar, centre Hildon. 

CEAATE RH 

