32 INTRODUCTION. 



and known as Gannister beds. Analyses are given in Binney's work just 

 cited. Herr Wedekind recently found nodules quite similar to the above 

 on refuse-heaps at Zeche Vollmond near Langendreer, but consisting of 

 dolomite, according to Weiss 1 , and not of calc-spar. The Fritz seam, 

 from which they seem to come, is unfortunately not worked at present ; 

 but in its roof occur fossils similar to those of the Gannister beds, and 

 the vegetable remains found in the nodules are quite the same as those 

 observed in England. Lastly, Stur 2 refers to similar nodules, peat-sphaero- 

 siderites, as he terms them, from the coal-seams of Witkowitz in Moravia and 

 from those of the upper Carboniferous deposits of Szekul in the Banat. The 

 analyses of the latter show that siderite and calc-spar are present together 

 in them in varying proportion. 



In Fifeshire, north of Edinburgh, are coal-bearing strata which lie 

 below the Millstone Grit, and are known by the collective name of the 

 Lower Burdie House Series. They correspond, according to Grand' Eury, 

 to the upper Kulmgrauwacke. In this series at Burntisland Grieve dis- 

 covered beds, alternating with volcanic tuffs, which consist of crystalline 

 calc-spar and are filled with vegetable remains exactly in the manner of 

 the petrified coals ; these beds have supplied much material for Williamson's 

 researches. 



Lastly, similar limestone beds occur at Laggan Bay in the Island of 

 Arran on the west coast of Scotland, which are full of well-preserved remains, 

 and, like the Fifeshire beds, lie between diabase-tuffs. Wiinsch 3 tells us that 

 a large number of stumps of trees, standing erect where they grew, were 

 discovered in his time in the compact black stone which takes a polish like 

 marble. The petrified outer portions of the stumps alone remain, the inside 

 having decayed and disappeared, and its place being taken usually by the 

 diabase-tuff which covers the whole deposit ; but the interior also of a few 

 stumps is filled with the petrifying material. 



The changes which petrifactions undergo from exposure to external 

 influences have been already noticed in more than one place. When they 

 are exposed to the air their organic substance is in most cases slowly 

 dissipated, and they may then have a transparent look and be colourless or 

 of a reddish hue, while when fresh from the place of deposit they are dark 

 brown or almost black. Water trickling over them naturally expedites the 

 process, and may even accomplish it before they have been exposed. This 

 change produced by atmospheric influences is particularly well shown in the 

 case of the silicified woods of the lignites of the districts of Meissner and 

 Zobten in Silesia, which when exposed to the air become encased in a 

 whitish rind. The organic substance may also be removed from petrifactions 



1 Weiss (2). - Stur _> . 3 Wiinsch (1). 



