464 Memoirs of Notices — Prof. James HalVs Faper. 



twelve of these trunks were found on two sides of an area less than 

 sixty feet square. They were of various sizes, from less than six 

 inches diameter in their smallest part to more than one foot in 

 diameter at the same height above the base. Several of them have 

 a diameter of more than two feet at the spreading base, and I have 

 seen one specimen of fully three feet in diameter at its base. 



The strata in the immediate neighbourhood contain few organic re- 

 mains except plants, but the strata both below and above as well as 

 on this horizon contain numerous fossil shells, Crustacea, etc. Going 

 in a westerly direction, the sandy beds lose their coarseness, and the 

 shales become finer, until we find deposits of calcareous mud. Ee- 

 ceding from what I suppose to have been the ancient shore-line, the 

 fossil shells are principally LameUibranchiata ; the BracJiiopoda do not 

 come in at all, or but in small degree, till we have travelled a con- 

 siderable distance to the westward. Moreover, when we have so far 

 left the shore-line that we can take satisfactory cognizance of the 

 condition of the sediments, we find them made up of alternations of 

 harder and softer beds — that the LameUibranchiata are confined to 

 the harder and coarser beds, and the BracJiiopoda to the finer sedi- 

 ments, as a rule. Not only so, but sometimes the coarser beds are 

 charged with a few species of particular genera, as of the Aviculo- 

 pecten, while others are crowded with Modiola-like forms, with few 

 Aviciilopecten ; while Cramm^/sia, a genus which may perhaps belong 

 to the TJnionidce has sometimes flourished abundantly to the almost 

 entire exclusion of everything else. 



Were we to make vertical sections at some points along a line 

 extending westerly from the first points indicated, we should have 

 something like what I have shown in the diagram where I have 

 indicated the harder layers as much thicker than the softer ones, 

 which are wedging out to the eastward. At another point, fifty or 

 one hundred miles westward, we should find many of the harder 

 beds wedging out in that direction, and the softer shales pre- 

 dominating in thickness, as shown in the diagram. 



It so happens that we have a direct line of outcrop of more 

 than three hundred miles from east to west, and while at the 

 eastern end the sediments are coarse, and the prevailing fossils are 

 LameUibranchiata, with a few large Cephalopods, we have, at the 

 western extremity, fine calcareous shales, with abundance of BracMo- 

 poda and Corals, while LameUibranchiata are rare. These for- 

 mations in their greatest thickness are quite four thousand feet, and 

 in their greatest attenuation about two thousand feet. Every layer 

 has, of course, at one time been the sea-bed on which the animals 

 have lived, and the final result has been the slow depression of this 

 sea-bed to accommodate the gradually accumulating sediment. 

 The belt of sediment in which the fossil trunks stand has again 

 been submerged, and hundreds of feet of marine strata of the same 

 age have accumulated above them. 



Now the question arises whether this depression has been gradual 

 and constant, or whether there have been intervals of depression, 

 and again of elevation, making the movement which resulted in the 



