in Yorkshire and Durham. 43 
the magnesian limestone; and the relations of all the analo- 
gous formations in the coal district seem to prove, that the 
Cockfield Fell dyke cannot pass out of the limits of the coal- 
formation. 
If we adopt this latter opinion, we must admit, that the dykes 
of Cockfield Fell and Cleveland (notwithstanding the agreement 
in their line of direction) belong to two distinct epochs. After 
all, the question is only one of local interest; and as far as 
regards the leading object of this paper, of no importance what- 
soever. 
Through the kindness of T. R. Underwood, Esq. of Paris, 
I have become acquainted with the results of an examination of 
specimens from several English trap dykes by Professor Cordier. 
I will subjoin his description of such specimens as were derived 
from localities alluded to in the preceding paper. 
No. 1. From Preston quarry in the Cleveland dyke. Mimosite, 
fine grained, imperfectly porpheroidal from the salient crystals 
of Pyroxéne. It is a Basalé of the ancient mineralogists. The 
specimen contains a great abundance of dark-greenish grey 
Felspar, mixed with a very small quantity of Pyroxéne and 
titaniferous Iron. Some points of Pyrites are to be seen. The 
paste also envelops laminar crystals of Felspar, having a con- 
siderable lustre, which give the paste a scaly appearance which 
distinguishes it from Basalt. 
No.2. From Coaly Hill dyke near Newcastle. Mimosite, 
small grammed, passing into Xerasite. Many of the cavities con- 
tain green-earth. It is imperfectly porpheroidal. The crystals of 
Felspar very brilliant. 
No. 3. From Walbottle Dean dyke. This has a more decided 
character of a Dolerite, very fine grained, the Felspar whiter 
than in the others. 
F2 
