^9 LECTURES ON GEOLOGY. 



and some of them contain abundant organic remains in a fossil or 

 petrified state. 



The circumstances which guide us in assigning certain relative 

 ages to different strata, are, the order of superposition, the differ- 

 ence of structure, the inchiding fragments of other rocks, and the 

 containing petrifactions of a greater or less number of organized 

 beings analogous to existing kinds. It is a remarkable fact that 

 the order of succession of stratified rocks is never changed, 

 although some of the strata may be absent ; but rocks of igneous 

 origin are often found piercing through the superincumbent strata, 

 ^o that with regard to these the criterion is not unerring. Struc- 

 ture is not a more certain guide than superposition, for although 

 crystallized rocks are usually older than sedimentary deposits, yet 

 shale and lias limestone have been converted into crystalline 

 marble and garnets (a primitive mineral) by their vicinity to 

 granitic and volcanic dykes, the impressions of the shells they 

 contained being all obliterated. It is evident, however, that a rock 

 containing fragments of other strata must be of more recent date 

 than these ; and by a comparison of their fossils we are enabled to 

 identify mineral masses from opposite sides of the earth as belong- 

 ing to the same era. 



Modern petrifactions are not really a conversion of the substance 

 into stone, but are merely incrustations of carbonate or sulphate 

 of lime, or of silica, upon any body submitted to the action of water 

 in which these minerals are dissolved. In this manner are formed 

 petrified bird's nests, in Derbyshire, and moss agates round the 

 basins of the boiliuii; springs in Iceland, as well as the medallions 

 of San Filippo, in Italy. In a true petrifaction, on the contrary, 

 the substance of the body is carried away, while solid matter, 

 usually crystalline, is deposited in its place. In some cases, how- 

 ever, the substance is removed, but nothing is deposited, so that a 

 vacant space, containing an impression of the body, only is left. 



The strata of the neighbourhood of Birmingham occupy a space 

 in the geological series from the old red sandstone to the lias, but 

 their description forms the subject of the succeeding lectures. It 

 is in the lias that the remains of those huge and strangely-formed 

 reptiles, the ichthyosauri and plesiosauri, first appear, and in the 

 contemporaneous formations of Germany that paradoxical ball-like 

 animal the pterodaclyte is found. In Portland Island, one of the 

 secondary formations, the curious phenomenon is observed of per- 

 fect plants allied to the cactus and palm tribes, standing erect in a 

 petrified state, with their roots fixed in a kind of soil, and their 

 trunks piercing into the superincumbent bed of limestone. The 

 occurrence of round flint stones in chalk, the last of the secondary 

 group, has been recently accounted for by what has been observed 

 to occur in potters' paste (a mixture of clay and ground flints) 

 "when allowed to settle. The flint separates from the clay into 

 nodules, which soon become exceedingly hard, and would probably 

 assume a crystalline form under favourable circumstances. The 

 chalk forms the limit of the secondary strata by the total differ- 



