1849.] BROWN ON THE SYDNEY COAL-FIELD. 127 



clays. The upper layers of the underclays immediately in contact 

 with the coal are argillaceous, forming sometimes good fireclays : from 

 six to twelve inches below the coal they contain variable proportions 

 of siliceous matter, being in a few instances scarcely distinguishable 

 from sandstones. 



Stigmarise are found in all the underclays, in several of the shale 

 roofs of the coal-seams, and in some beds of shale (Nos. 81, 238 and 

 274) which constitute neither floors nor roofs. In the first case they 

 are very numerous, and of several species, although S. ficoides is the 

 most common. In the second they are not so plentiful, occurring pro- 

 bably only in connexion with the erect trees, of which they are the 

 roots. In the last case they are very rare, and where they are not 

 united to erect trees, most probably have been drifted, carrying with 

 them a portion of their long rootlets, of which there is one very de- 

 cided example in another part of the Sydney coal-field *. 



It does not appear that the thickest underclays, containing the 

 greatest number of Stigmarise, are overlaid by proportionably thick 

 seams of coal ; the reverse is often the fact : for instance, the three 

 lowest seams in our section (Nos. 8, 12 and 15), none of which ex- 

 ceed 1 inch, lie upon underclays 4 ft. 10 in., 5 ft. 4 in. and 3 ft. 4 in. 

 in thickness, respectively, while on the other hand, the seams Nos. 

 78 and 295, the one 4 ft. 8 in. and the other 5 ft., rest upon under- 

 clays by no means rich in Stigmarise, only 2 ft. 3 in. and 3 ft. 4 in. 

 in thickness. In the two latter cases, probably, the increase of Stig- 

 marise was interrupted by a subsidence which produced the conditions 

 required for the growth or accumulation of the vegetable mass from 

 which the coal beds were formed. In the former instances, similar 

 conditions having been obtained and the growth of the vegetable 

 mass just commenced, a subsidence took place almost immediately, 

 which at once put a stop to the increase of the material required 

 to form coal. If the strata had been suffered to remain in a state 

 of rest for a longer period, the bed of coal No. 8, which is only half 

 an inch, might have attained a thickness equal or superior to any in 

 the section. 



As before mentioned, the seam of coal No. 27 forms an exception 

 to the general rule ; it lies upon a coarse limestone 6 inches thick, 

 which rests upon a hard arenaceous shale, not unlike some of the more 

 sandy underclays ; but it neither crumbles on exposure to the weather, 

 nor does it contain the slightest trace of a Stigmaria or its rootlet. 

 As the Hmestone contains Cyprides, with the scales and coprolites of 

 fishes, which could only have been deposited in water apparently of a 

 very moderate depth f , we must admit that the thin layers of coal 



* Whilst examining the cHffs about three years ago near the entrance of the 

 Brasd'or lake, accompanied by Prof. Johnson of Philadelphia, we found a piece of 

 Stigmaria 6 inches long and 3 inches in diameter, with rootlets attached, in the 

 middle of an upright fossil tree. This could only have been drifted : probably 

 those found in the shales numbered 81, 238, &c. were drifted in the same way. 



t The depth of the water could not have exceeded 3 feet, because at that 

 height above the coal we find a soil (underclay) containing the roots (Stigmariae) 

 of trees which once grew on its surface, unless we assume that the strata had been 

 elevated after the deposition of the coal. 



