62 G. C. Crick — On Cyrtoceras from Derby sliire. 



Miocene. Nor is it likely that a basalt outburst on one side of the 

 Nile Valley should be of Lower Oligocene age, while a similar flow 

 of lava on the other side is of Lower Miocene age, yet this is what 

 Dr. Blanckenhorn states in his work previously quoted. 



From what has been said, however, it would appear that a definite 

 sequence of events has been established, and the following summary 

 may now be made of the conclusions arrived at by the writer. 



1. The Petrified Forest has not been silicified in situ ; the trees 

 have been brought from a distance by a river and deposited in 

 a shallow lagoon or estuary, 



2. The trees were not silicified immediately on deposition ; they 

 lay embedded in the sands for some time and underwent some 

 decomposition, as is witnessed by the total absence of bark, branches, 

 and leaves, and by the irregular and pitted exterior of the trunks, 

 which must represent the external condition of the trees when the 

 petrifying waters reached them. 



3. After the deposition of the sands in which the trees were 

 embedded had practically ceased, there was an outbreak of basalt 

 which covered a good part of the beds. 



4. After the volcanic action had ceased, earth-movements produced 

 fissures up which warm si Heated waters rose in the form of geysers 

 and hot springs. The volcanic necks were fractured, and eventually 

 the fissures thus produced were filled up by plugs of quartzite or 

 chalcedonic silica, the surrounding rocks were thrown into shallow 

 basins by two sets of folds, and the silicated water which collected 

 in these basins silicified the trees lying in the sands. 



5. When the geysers and hot springs had died out, a good deal of 

 denudation took place ; the hard plugs representing the throats of 

 the springs were left standing out in relief; and when the Lower 

 Miocene sea (first Mediterranean) invaded this district from the 

 north-west, it deposited its sediments round these stumps and pro- 

 duced the appearance which has led some observers to the conclusion 

 that the geysers occurred at that period. 



6. The age of the Petrified Forest, the Gebel Ahmar Sands and 

 Sandstone, and the associated lava-flows, is undoubtedly Oligocene, 

 as these deposits fall between Eocene and Lower Miocene beds. 



V. — On a specimen of Cyrtoceras (Melooeras) apicale from 

 THE Carboniferous Limestone, Kniveton, Derbyshire. 



By G. C. Crick, F.G.S., of the British Museum (Natural History). 



(PLATE III.) 



rpHE specimen which forms the subject of the present com- 

 JL munication is a fragment of a Nautiloid in the collection of 

 Dr. Wheelton Hind, F.G.S., from the Carboniferous Limestone, 

 Kniveton, Derbyshire, that seems to be referable to Cyrtoceras 

 (Meloceras) apicale. This species, instituted by Dr. Foord in 1898,^ 



1 Monograph of the Cai-boniferous Cephalopoda of Ireland (Palseontographical 

 Society), pt. ii, 1898, pp. 33-36, pi. xi, figs, la, lb, 2a, 2b, 3 ; pi. xii. 



