570 ESTUARY DEPOSITS SHOWING HOW COAL MAY HAVE BEEN FORMED. 
Modern Estuary Phenomenon explanatory of the former origin of Coal. — Besides 
these proofs of successive desiccation and change of outline, the North of Russia 
affords examples of modern accumulations, some of which throw light upon the 
probable origin, in more ancient epochs, of certain beds of coal. The embouchure 
of the Dwina, from ten or twelve miles broad at Archangel, is studded by not less 
than 250 small wooded islets, the sides of which and of the low country on the 
left bank of the stream, rise only to the height of a few feet above the high water- 
mark, and exhibit numerous alternations of fine silt. On disembarking at that 
bank near the post station opposite to the city, we found the cliff to be composed 
of the following materials in descending order : — 1st, vegetable soil and boggy 
woodland, into which the roots of living trees penetrated ; 2nd, clay and sand, 
alternating in fine laminae, with fragments of decayed wood, and indicating the 
deposit by the river ; 3rd, bog and peat, the remains of a former decayed vegetation, 
with blackened and rotted roots, &c . ; 4th, river sand repeated ; 5th, stiff blue clay, 
reaching down to the water’s edge. Now this arrangement seemed to us very 
distinctly to indicate the alternation of river freshes or inundations with periods 
of dry land, on which vegetables grew, whilst the blue clay or base of the section 
might represent the ancient bottom of the estuary, contemporaneous with that in 
which, higher up the Dwina, we had found the post-pliocene shells. At all events, 
whatever the lower blue clay might be, the overlying beds offer all the analogy 
which we require, in order to account for the phenomena prevalent in some of our 
coal-fields, of the alternation of certain beds of coal and shale, wherein all the 
vegetables present the appearance of having been entombed in situ with other 
large layers, indicating the action of drift. For if this low left bank were sub- 
merged, and its materials consolidated by long-continued pressure, we might, 
doubtless, anticipate that there would be produced two distinct carbonaceous 
masses, one, in fact, formed out of vegetation in place, whilst the other, composed 
of estuary silt, and converted into carbonaceous sandstones and shale, would con- 
tain, here and there, fossil stems of trees which had been drifted by the stream, 
and placed irregularly, either athwart the strata, or laid along them in flattened 
masses. 
Modern Ravines or “Avrachs.”— There are no superficial features in Russia more 
worthy of the notice of geologists, than the striking fissures which are from year to 
year laid open in the earth, and often proceed to great depths downwards, not 
only into the drift and ancient alluvia, but also into the true subsoil. Some of 
