360 
Directions for making Geological 
[Deg. 
forward, and the penetration of water containing oxide of iron. It is generally 
remarked, that in rising from the primitive to the tertiary rocks, quartz becomes 
a less prevalent ingredient in their composition. Quartz or silex is, therefore, 
considered indicative of older rocks or formations ; but the chalcedonic beds, in 
the upper fresh water formation at Paris, is an exception to this, since from its 
situation it must be referred to a very recent period of formation. 
The occurrence of a conglomerate breccia or puddingstone is considered indica- 
tive of the epoch of a new order or class of rocks or formation, or when the 
upper beds are un conformable to those on which they rest. It is from the organic 
remains, particularly the shells, that the different epochs of geology are distinguished 
The red marl beds which lie between the coal and the lias are considered a marked 
point in the series; the shells found in the beds above being different from those 
found in the beds below. Some of the shells, however, as the Nautilus and Ammo- 
nites, are found through a great range, and the remains of a land animal have been 
lately found in the oolite beds, in the stonesfield slate, (fig. 6,) previously to 
which no such remains were considered to exist lower than the chalk. It is only 
when in beds at different points on the earth’s surface, similar shells are found in 
considerable quantity, that these beds can be considered as belonging to the same 
period of formation ; thus in the lower marine formation of the Paris basin (the 
calcaire grossiere, fig. 8) and the London clay, there are about one hundred shells 
common to both. 
It will be seen that beds may be considered in this point of view as geologically 
the same, although different in their mineralogical composition ; and it may here 
he remarked, that besides the unsatisfactory definition of several of the species of 
rocks which was given in their description, there are many beds in the secondary 
and tertiary portions of the series, which aie so mixed and indistinct, as scarcely to 
admit of any description of their mineralogical characters. These beds are traced 
and recognized at distant points, chiefly from their geological connexion, or their 
place in the series in relation to each other, and to well established points in it, or 
from the shells which they contain, as also from their own internal character, 
and that of their accompanying beds. 
From such indistinct characters, many of these beds have been traced over 
a great portion of England, as will be seen in the papers and map of the Geologi- 
cal Society. The lias and chalk are considered the best established points in the 
secondary series, to which therefore the other beds are referred. 
The red marl above mentioned contains the deposits of salt and gypsum, and 
these are always included in it, wherever it lias been found on the earth’s surface. 
It is this, and similar uniformity being found to exist, which gives to geology its 
great interest, and points out the advantages to he derived from it in directing future 
researches : for the positions which the strata occupy in the series are always found 
to be the same, at least they are never reversed, though some of the members of 
the series may be wanting. Thus, in Sutherland, an instance is mentioned of coal 
being separated from granite by only a thin stratum of conglomerate ; and in 
France and Boulogne the oolite is fouud resting on the mountain limestone. In 
Ireland also, the chalk is stated to be found resting immediately on the sandstone 
which covers the coal measures ; all the oolite and lias beds which are found to in- 
tervene in the English strata (as seen in the section, fig. 6. 7) being wanting; another 
situation i3 mentioned near Fairhead, where the basalt comes immediately in con- 
tact with the coal measures. The detail and description of the observations above 
alluded to, will be found in the 3d vol. of the Geological Transactions, in a paper 
by Dr. Berger and Mr. Conybeare. In that paper it is mentioned, that the bed 
corresponding in its fossils to the English greensand, is found in its place in Ire- 
land, namely, under the chalk, where it is denominated mulatto stone : it is de- 
scrned as a calcareous sandstone, mixed with specks of green earth and oxide of 
non. The lias limestone is also said, in the same paper, to be found at different 
points in the north of Ireland, alternating with clay slate : it is described as a blue 
argillaceous limestone, similar in appearance to the English, and containing a great 
many shells of Ammonites, Gryphites, and Pentacrinus. The red marl lias likewise 
^ j**’ ant * * ts conf d ant subordinate portions of gypsum and sa(t ; but the 
I / 0 appear, as far as these observations have been made, to be wanting in 
direct n - Gcolo nj sts who have visited the north of Ireland, have, in particular, 
basalt * • 1 1C T, attent ' on to the whin dykes, and to the junction of the chalk and 
Pacer ■ V- )e S * ?ea * n ^ le Phdes to the paper above mentioned, and also in a 
many obser * . tcliardson, published in the Irish Transactions, vol. 9. There are 
*ion attend 1 10ns ®* ven » which can only be explained from the force and convul- 
an on the formation of the basalt. In some places, masses of chalk* 
