Part 2 (1/2)
In short, the formation consists of masses of sandstone, shale, li clays and ironstones, and, in the limestone, marbles and veins of the ores of lead, zinc, and antimony, and occasionally silver
[Illustration: FIG 18--Sigillarian trunks in current-bedded sandstone
St Etienne]
As the most apparent of the rocks of the system are sandstone, shale, limestone, and coal, it will be necessary to consider how these were deposited in the waters of the carboniferous ages, and this we can best do by considering the laws under which strata of a si deposited as sedireat proportion consists of sandstone Now sandstone is the result of sand which has been deposited in large quantities, having becoht to bear upon it It is necessary, therefore, first to ascertain whence came the sand, and whether there are any peculiarities in its method of deposition which will explain its stratification It will be noticed at once that it bears a considerable a,”
that is to say, that the strata, instead of being regularly deposited, exhibit series of wedge-shapedout
Sand and quartz are of the same chemical composition, and in all probability the sand of which every sandstone in existence is composed, appeared on this earth in its first solid form in the shape of quartz
Now quartz is a comparatively heavy mineral, so also, therefore, will sand be It is also very hard, and in these two respects it differs entirely from another product of sedimentary deposition, namely, mud or clay, hich we shall have presently to deal when co to the shales Since quartz is a hard mineral it necessarily follows that it will suffer, without being greatly affected, a far greater a transported by the agency of currents and rivers, than will a softer substance, such as clay An equal a action upon clay will reduce it to a fine irains of sand, however, will still ree size, and where both sand and clay are being transported to the sea in one and the sa distances, whilst the sand, being heavier, bulk for bulk, and also consisting of grains larger in size than grains of clay, will be rapidly deposited, and form beds of sand Of course, if the current be a violent one, the sand is transported, not by being held in suspension, but rather by being pushed along the bed of the river; such an action will then tend to cause the sand to become powdered into still finer sand
When a river enters the sea it soon loses its individuality; it becoed in the body of the ocean, where it loses its current, and where therefore it has no power to keep in suspension the sediher lands When this is the case, the sand borne in suspension is the first to be deposited, and this accumulates in banks near the entrance of the river into the sea We will suppose, for illustration, that a sradually approaches the sea, and the current loses its force, the sand is the , until finally it falls to the bottom, and forms a layer of sand there This layer increases in thickness until it causes the depth of water above it to beco place, the current will still have a certain, though slighter, hold on the sand in suspension, and will transport it yet a little further seaward, when it will be thron, at the edge of the bank or layer already for to extend the bank, and to shalloider space of river-bed
As a result of this action, strata would be foronally as well as horizontally, represented in section as a nuly been thron one above the other, ending in thin wedge-shaped terminations where the particular supply of sediment to which each owed its formation had failed
The masses of sandstone which are found in the carboniferous fore-shaped strata, and we have therefore a clue at once, both as to their propinquity to sea and land, and also as to the manner in which they were formed
[Illustration: FIG 19--_Productus_ Coal- more, too, about the, we could observe that the wedge-shaped strata always pointed away from the source of the e that in the carboniferous strata the sa strata were for out was siood, however, by a further deposition of strata when the next supply was borne down
It is scarcely likely, however, that sand in a pure state was always carried down by the currents to the sea Sometimes there would be soe masses of almost pure sandstone have been formed, so in other places shales, or, as they are popularly known by miners, ”bind,” have been formed Shales are formed from the clays which have been carried down by the rivers in the shape of silt, but which have since become hardened, and now split up easily into thin parallel layers The reader has no doubt often handled a piece of hard clay when fresh from the quarry, and has re it up, in order, perhaps, to excavate a partially-hidden fossil, it has readily split up in thin flakes or layers of shaly substance This exhibits, on a small scale, the chief peculiarity of the coal shales
The formation of shales will now de doith it a quantity of mud or clay, it is transported as a fine, dusty silt, and when present in quantities, gives the muddy tint to the water which is so noticeable We can very well see how that silt will be carried down in greater quantities than sand, since nearly all rivers in soh a clayey district, and finely-divided clay, being of a very light nature, will be carried forhenever a river passes over such a district And a very slight current being sufficient to carry it in a state of suspension, it follows that it will have little opportunity of falling to the bottom, until, by some means or other, the current, which is the means of its conveyance, becomes stopped or hindered considerably in its flow
When the river enters a large body of water, such as the ocean or a lake, in losing its individuality, it loses also the velocity of its current, and the silt tends to sink down to the botto less heavy than the sand, about which we have previously spoken, it does not sink all at once, but partly with the iht velocity which the current still retains, even after having entered the sea, it will be carried out soradually sink to the bottoreater the possibility of its drifting farther still, since in sinking, it would fall, not vertically, but rather as the drops of rain in a shohen being driven before a gale of wind Thus we should notice that clays and shales would exhibit a regularity and uniformity of deposition over a wide area Currents and tides in the sea or lake would tend still further to retard deposition, whilst any stoppages in the supply of silt which took place would give the former layer tiiving it that bedded structure which is so noticeable in the shales, and which causes it to split up into fine laminae This uniformity of structure in the shales over wide areas is a well ascertained characteristic of the coal-shales, and we iven here with a degree of certainty
There is a class of deposit found a the coal-beds, which is known as the ”underclay,” and this is the ular of all as to the position in which it is found The underclays are found beneath every bed of coal
”Warrant,” ”spavin,” and ”gannister” are local na a tere proportion of silicious matter as to become almost like a hard flinty rock Sometimes, however, it is a soft clay, at others it is mixed with sand, but whatever the co unstratified They also agree in this respect that the peculiar fossils known as _stigmariae_ abound in them, and in some cases to such an extent that the clay is one thickly-matted mass of the filaradually carew in this age, and whose remains have subsequently become metamorphosed into coal, and it is but one step farther to come to the conclusion that these underclays are the ancient soils in which the plants grew
No sketch of the various beds which go to form the coal-measures would be complete which did not take into account the enormous beds of mountain limestone which form the basis of the whole systest the upper portion of the system, or the true coal-measures
Now, limestones are not formed in the same way in which we have seen that sandstones and shales are forin to their deposition as sediment in seas, estuaries or lakes, but the ical forin to causes other than that of sedimentary deposition
In carboniferous times there lived numberless creatures which we knoadays as _encrinites_ These, when growing, were fixed to the bed of the ocean, and extended upward in the shape of pliant stems composed of limestone joints or plates; the stem of each encrinite then expanded at the top in the shape of a gorgeous and graceful starfish, possessed of nurew in such profusion that after death, when the plates of which their stems consisted, became loosened and scattered over the bed of the sea, they accumulated and formed solid beds of limestone Besides the encrinites, there were of course other creatures which were able to create the hard parts of their structures by withdrawing lime from the sea, such as _foraminifera_, shell-fish, and especially corals, so that all these assisted after death in the accurown and lived
[Illustration: FIG 20--Encrinite]
[Illustration: FIG 21--Encrinital limestone]
There is one peculiarity in connection with the habitats of the encrinites and corals which goes so us with a useful clue as to the conditions under which this portion of the carboniferous formation was formed These creatures find it a difficult matter, as a rule, to live and secrete their calcareous skeleton in any water but that which is clear, and free from enerally speaking, where the other deposits which we have considered, are for, and, as these are always found near the coasts, it follows that the habitats of the creatures referred to must be far out at sea where no muddy sediments, borne by rivers, can reach them We can therefore safely coe masses of encrinital limestone, which attain such an enormous thickness in some places, especially in Ireland, have been formed far away from the land of the period; we can at the same time draw the conclusion that if we find the encrinites broken and snapped asunder, and the liled with a proportion of clayey or sandy deposits, that we are approaching a coast-line where perhaps a river opened out, and where it destroyed the growth of encrinites,with their dead remains the sedimentary debris of the land