Geology of High Teesdale. 185 
de les confondre avec les argiles schisteuses ou les schistes 
argileux,” Essai, p. 249. He afterwards adds, “‘ Z’on peut donc 
conclure que ces roches feldspathiques sont liées intimement aux 
dépéts basaltiques,” Se. §e. p. 252. To such remarks as these, 
I would observe by way of reply—(1.) That hand specimens of 
slate-clay, indurated by the action of trap, sometimes resemble 
true Lydian-stone which is subordinate to transition-slate, but 
may generally be separated from it by the help only of external 
characters. (2.) That the fusibility of such indurated specimens 
into a light coloured enamel, or into glass of various shades between 
light grey and brownish black, affords no test whatsoever by which 
we can separate them geologically from the soft unaltered beds 
of slate-clay, and arrange them with feldspathic rocks of a different 
family ; because specimens from the soft unaltered argillaceous 
beds (especially those which alternate with limestone and con- 
tain a portion of calcareous matter) generally melt into an enamel, 
and sometimes into a light transparent glass. Lastly, the hypo- 
thesis of Boué affords no solution of, and is contradicted by, 
the phenomena which are so constantly seen in the vicinity of 
trap dykes*. 
His account of the trap dykes of the Northumberland and La Pcla 
Durham coal-fields, is, throughout, either defective or erroneous. 
For example: he states that some of those dykes are separated 
from the neighbourmg rocks, by a salbande of grit, containing 
vegetable fossils. There is no such salbande; and he has been 
led into the mistake, by misinterpreting a passage in the paper 
of Mr. Winch +. 
* In confirmation of this assertion, I may refer to passages without number in 
Dr. Mac Culloch’s Geological Description of the Hebrides; to Professor Henslow’s Paper 
on the Geology of Anglesea; and to certain passages in the description of the trap 
dykes in the Durham coal-field (supra, p. 35, &c.) 
+ Geological Transactions, Vol. IV. p. 21. 
Vol. II. Part I. AA 
