﻿PRESERVATION AND EXTERNAL CHARACTERS. 2T, 



LOCAL CONDITIONS OF PRESERVATION. 



In the Potomac Formation of Maryland, so far as determined from the sur- 

 roundings, the cycad trunks now found isolated were embedded in the " iron-ore 

 clays," often somewhat arenaceous and generally supposed to be of brackish-water 

 origin (5 a). These trunks are not nearly so well silicified as those of either the 

 Black Hills or Wyoming, although externally of much beauty. (See figure 1, p. 

 4, and 7, p. 32.) It is quite probable that the Maryland trunks were all subjected 

 to more or less maceration, under estuarine conditions, although rather better pre- 

 served than the most of the trunks from the Purbeck beds of the Isle of Portland, 

 where the "crows' nest" forms, or trunks with decayed and crushed summits, are 

 probably often found in the original position in which they grew. 



In the Black Hills Rim there are, as has been stated in the chapter on dis- 

 tribution, two clearly distinct cycad-bearing horizons. The lower of these hori- 

 zons is doubtless equivalent to that of the Freezeout Hills of Carbon County, 

 Wyoming, and lies near the base of the shales and sandstones of the fresh-water 

 Jura or Atlantosaurus beds of Marsh. The cycads of this lower level weather out 

 of the shales, so far as kuown, together with much silicified coniferous wood which 

 has, in common with the cycads, a very characteristic habit of weathering dirty 

 white, and like them shows a very dark surface when freshly fractured. Both are 

 dense of texture and must be studied from very thin sections. Fvidently they grew 

 in much the same situations and were preserved at the same time and in relatively 

 the same manner. In the cycads in particular there is considerable evidence of 

 maceration. The trunks are quite frequently much flattened, and the armor is often 

 crushed out of position and very variably preserved, as the direct result, no doubt, 

 of compression, while the siliceous material replacing the original plant tissues was 

 still in a more or less gelatinous condition. Yet, although far from having the 

 symmetrical appearance of the Maryland trunks, the differentiation of tissues is 

 often of much greater and rare beauty. The copious growth of ramentum was 

 particularlv susceptible to silicification, and in some instances, as will be seen, the 

 structural details approach and in some respects surpass, in distinctness and perfec- 

 tion, those of the thin sections from similar tissues of living plants. Of course, in 

 making the latter we are at a disadvantage because of the difficulty of firmly embed- 

 ding woody, woolly, or scaly material for the cutting of thin sections of consider- 

 able area. It is not improbable that there was a more abundant source of alkaline 

 silicate solutions than in the iron-ore beds of Maryland — some steadier flow as well 

 as the presence of more iron. So far as yet determined, in addition to the bones 

 of Dinosaurs, the plants of the Atlantosaurus beds, including large logs and the 

 cycads, were brought by streams into the ox-bows and estuaries about a large and, 

 as has been commonly supposed, deep fresh-water lake. Whether or not the plants 

 were immediately silicified is, of course, difficult to say, although it is probable that 

 chemical action was not long deferred. The required silica may perchance have 

 been derived from siliceous ash of volcanic origin, from diatoms, from siliceous 

 sponges, or from deep-seated thermal waters, the former presence of which is sug- 

 gested by certain siliceous cores the writer has observed projecting from the shales 



