212 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. (Jan. 19, 
the red conglomerate beds dipped, was the lowest of these igneous 
rocks, and therefore probably constituted the nucleus of the vol- 
canic action of this portion of the district. 
About a mile further to the N.W., after crossing the valley in a 
diagonal direction, is the little village of Idal, immediately beyond 
which the hill rises gradually to the W. and N.W. It is in the upper 
portion of these hills that the principal agate quarries are found. Near 
the summit is an escarpment fifty feet in height, facmg the W.N.W. 
The formation of which the hill consists is a greenish-brown trap 
rock, with a slight appearance of irregular stratification in that 
portion of it which contains the agates, dipping, if it can be so called, 
at a very slight angle to the E.S.EK., viz. towards Idal. It here 
consists of two varieties alternating with each other in beds varymg 
from two to four or five feet in thickness, and it is, in fact, this alter- 
nation which gives the appearance of real stratification. One of 
these varieties is much softer than the other, and of a more amyg- 
daloidal character. It contains numerous irregularly-compressed, 
almond-shaped nodules, varying in size from an inch to a foot in 
length, most if not all of which, on being broken, prove to be chal- 
cedony or agates. It is worthy of observation, that the length of the 
nodules corresponds with the inclination of the bed, and that their 
direction is always parallel, thus suggesting the idea that the 
lengthening-out of the cavities has been owing to pressure while the 
mass was still in a viscous state. 
The other stratum is of a much harder nature, more compact, and 
with a more decided cleavage ; this however is very irregular, and 
resembles what may be called wedge-shaped cleavage. The colour, 
too, is generally browner than that of the former, showing a certain 
degree of yellow ochreous oxydation on the exposed surfaces, but its 
most remarkable feature is the total absence of all amygdaloidal cha- 
racter ; it contains no nodules whatever, neither agates nor con- 
eretions of siliceous or other matter. | 
The quarries themselves are opened in the escarpment above- 
mentioned, and penetrate some distance into the rock, keeping as 
much as possible in the softer beds containmg the nodules, and 
therefore descending slightly with the imclination of the mass. In 
some of these quarries, where however no work was then going on, 
I observed several alternations of the two varieties of trap formation 
which I have just described, but could not distinguish any marked — 
line or separation between them, and they seemed to pass gradually 
into each other. 1 was unable to arrive at any satisfactory explana- 
tion of this difference of appearance. The igneous character of the 
rock is too apparent to allow us to conclude that this is real strati- 
fication, and the complete union of the beds precludes the idea of 
their being different coulées successively poured forth over each 
other. Even supposing them to be altered metamorphic rocks, the 
fusion which they have undergone has been so complete as to have 
entirely destroyed all trace of stratification, and to have reduced the 
whole mass to one homogeneous paste. Yet how came this remark- 
able alternation? It may be owing to some chemical or perhaps 
