352 G. W. Bidman — On Uiiderclai/s. 



a few exceptions — to quiet and deepish M'aters. Thus the formation 

 of clay is described as follows in Phillip's " Manual of Geology " : 



"The carbonic acid, which is always dissolved in water, attacks 

 the felspar by dissolving out from it carbonates of potash, soda, or 

 lime; and then the crystals lose their hardness and become changed 

 into a paste of impalpable fine particles which form a mud. This 

 mud is held in suspension longer than the sand, and is therefore 

 carried further out to sea. When it falls to the bottom and is 

 compressed by the weight of the water above, it becomes clay, and 

 margins or surrounds the land as an outer belt, probably twice as 

 broad as the sand belt. On some coasts, like the South American 

 coast mentioned by Mr. Darwin, there may be no clay deposited 

 within one hundred and fifty miles of land " (vol. i. p. 50). 



Sir Charles Lyell, again, describes the deposition of clay as taking 

 place in the deeper portions of Lake Superior. And one of the 

 results of the " Challenger " expedition has been to show that 

 oceanic deposits in depths greater than 2000 fathoms are clays. 

 But satisfactory cases of deposits at all analogous to underclays 

 being laid down in a shallowing area just before vegetation com- 

 mences to grow in the present order of things do not seem to be 

 forthcoming. And if underclays were so deposited why are they 

 not laminated ? 



That the geological study of clays is behind that of other rocks, 

 while satisfactory evidence from causes now in operation is not 

 forthcoming, seems to be indicated in the following quotation from 

 Phillip's Manual (part i. ed. 1885) : 



"No such careful and detailed examination has been made of 

 existing mud and clay as of sand or limestone. The subject is 

 much more difficult .... and the observations on deposits now 

 forming are too few to completely demonstrate the conditions under 

 which many of the newer clay beds were formed." 



In the following description by Prof. Green of the formation of 

 coal there is no room left for the deposit of the underclay : 



" But the downward movement was not without intermission ; 

 every now and then a pause occurred, and whenever this happened 

 the water would tend to be filled up. Sand-banks formed shoals, 

 M'hich by degrees grew into islands : the channels between the 

 islands were gradually choked up with sediment, till at last the 

 whole or portions of the lake became replaced by swampy plains 

 or morasses, across which sluggish rivers slowly wound their way. 

 Vegetation spread from the adjoining land over the marshj' flats, 

 and under the moist and otherwise favourable conditions grew apace, 

 till the whole became converted into a rank and tangled jungle " 

 (Coal, its History and Uses, pp. 56-7). 



According to this view the coal is evidently formed on a sand-bank. 



The analogy of modern peat bogs, which are generally found to 

 be underlaid by some sort of clay, is sometimes strongly insisted 

 upon in connection with underclays ; and yet, on examination, it 

 appears that many of our peat bogs rest on clays belonging to a 

 different geological age — as on the Boulder-clay, or on the clays of 

 the Coal-measures themselves — and so do not admit of comjDarison 



