VARIOUS KINDS OF FOSSIL PLANTS 15 



the case of those which are very well preserved this 

 must have taken place with considerable rapidity, so 

 that they were shut away from contact with the air and 

 from the decay which it induces. 



Impressions in the thin sheets of fine rock may be 

 compared to dried specimens pressed between sheets 

 of blotting paper; they are flattened, preserved from 

 decay, and their detailed outline is retained. Fossils 

 of this kind are most valuable, for they give a clear 

 picture of the form of the foliage, and when, as some- 

 times happens, large masses of leaves, or branches 

 with several leaves attached to them, are preserved 

 together, it is possible to reconstruct the plant from 

 them. ■ It is chiefly from such impressions that the in- 

 spiration is drawn for those semi-imaginary pictures of 

 the forests of long ago. From them also are drawn 

 many facts of prime importance to scientists about the 

 nature and appearance of plants, of which the internal 

 anatomy is known from other specimens, and also about 

 the connection of various parts with each other. 



Sometimes isolated impressions are found in clay 

 balls or nodules. When the latter are split open they 

 may show as a centre or nucleus a leaf or cone, round 

 which the nodule has collected. In such cases the 

 plant is often preserved without compression, and may 

 show something of the minute details of organization. 

 The preservation, however, is generally far from perfect 

 when viewed from a microscopical standpoint. Fig. 9 

 shows one of these smooth, clayey nodules split open, 

 and within it the cone which formed its centre, also 

 split into two, and standing in high relief, with its 

 scales showing clearly. Similar nodules or balls of 

 clay are found to-day, forming in slowly running water, 

 and it may be generally observed that they collect 

 round some rubbish, shell, or plant fragment. These 

 nodules are particularly well seen nowadays in the 

 mouth of the Clyde, where they are formed with great 

 rapidity. 



