218 W. H. Penning — On Concretions, 



an unconformity is of course shown. Because of the unevenness 

 of the sea-bottom at the commencement of the Carboniferous 

 epoch, it is difficult to estimate the amount of the depression 

 which now took place ; but since the sediments belonging to this 

 period in some cases are as much as from 15,000 to 18,000 feet in 

 thickness, it is evident that it must have been very considerable in 

 extent. This period of gradual subsidence was followed again by 

 great movements in the earth's crust, accompanied in many cases by 

 violent volcanic action. Enormous areas were now upheaved to a 

 great height, and mountainous ranges formed. The depression 

 which had been gradually taking place over so large an expanse of 

 the Northern Hemisphere seemed also at last to have reached its 

 limit, and the land surface was now restored, partly in its pro- 

 portion, though not in its general appearance, to the state it was in 

 at the close of the Laurentian, or before the Cambrian rocks were 

 deposited. Three periods of great disturbance occurred in Palaeozoic 

 time, and they were progressively more violent in their effects. 

 Each also followed epochs of quiet and gradual depression, and they 

 were most marked in the areas first submerged, where most sedi- 

 ment had accumulated, and hence where the pre-Cambrian crust had 

 apparently become thinnest. The depression in some places for the 

 whole of the Paleeozoic could not have been less altogether than 

 50,000 feet, and conformable sediments to that extent are now found 

 in some of the areas first submerged, and which remained undis- 

 turbed. In the south-west of Wales, where they attain this thick- 

 ness, the same folds affect the whole series, from the Cambrian to 

 the Carboniferous, and it is evident that in this region these deposits 

 are all perfectly conformable to one another, though so near to the 

 disturbed area of North Wales. 



Along the eastern borders of North America (xippalachian 

 region, etc.) the average thickness of conformable sediments, accord- 

 ing to Hall, Dana, etc., is not less than from 40,000 to 45,000 feet ; 

 therefore the depression there must also have been nearly as great 

 as that which took place in the western borders of Europe. 

 (To be concluded in our next Number.) 



Y. — Concretions. 



By W. H. Penning, F.G.S., 



Of H.M. Geological Survey of England. 



THEEE is a fine exposure of the " Lower Greensand " in a rail- 

 way cutting by Sandy Station on the G.N. and L. & N.W. 

 Eailways, presenting a nearly vertical section 50 or 60 feet in 

 height. It consists almost entirely of clean sand, in parts false- 

 bedded, and more or less coloured throughout by the presence of 

 oxide of iron. The colour thus imparted to the sand varies from 

 almost pure white, at the lower part, through shades of grey and 

 yellow, to a deep rusty brown. There are here and there layers of 

 hard ferruginous concretions in tabular, spherical, and many other 

 forms, presenting a feature hitherto (so far as the writer is aware) 



