THE MESOZOIC AGES. 195 



Was the length of the Mesozoic time equal to that 

 of the Palaeozoic ? Measured by recurring cycles it 

 was. In the latter period we find five great cycles, 

 from the Lower Silurian to the Permian inclusive. 

 So in the Mesozoic we have five also, from the Trias 

 to the Cretaceous inclusive. We have a right to 

 reckon these cycles as ages or great years of the 

 earth ; and so reckoning them, the Mesozoic time may 

 have been as long as the PalaBozoic. But if we take 

 another criterion the result will be difi'erent. The 

 thickness of the deposits in the Palaeozoic as com- 

 pared with the Mesozoic, where these are severally 

 best developed, may be estimated as at least four or 

 five to one; so that if we suppose the beds to have 

 been formed with equal rapidity in the two great 

 periods, then the older of the two was between four 

 and five times as long as the latter, which would 

 indeed be only a little greater than one of the separate 

 ages of the Palaeozoic. Either, therefore, the deposits 

 took place with greater rapidity in the Palaeozoic, or 

 that period was by much the longer of the two. This 

 it will be observed, is only another aspect of the great 

 laws of geological sequence referred to in our last 

 paper. 



Let us look into this question a little more minutely. 

 If the several pulsations of our continents depended 

 upon any regularly recurring astronomical or terres- 

 trial change, then they must represent, at least 

 approximately, equal portions of time, and this, if 

 proved, would settle the question in favour of an 



