30 



SCIENCE 



[N. S. Vol. XXXIII. No. 



Trees surrounding the marsh have been killed 

 and are now standing among salt marsh 

 grasses. Since breaches in bars, widening of 

 tidal inlets and other shore-line changes may 

 cause local fluctuations of a number of feet 

 in the high-tide surface, Professor Johnson 

 hastily concludes that no evidence of recent 

 subsidence on the New England coast thus 

 far presented can be considered satisfactory. 



It is the mean sea level, rather than the 

 irregular and changeable high-tide surface, 

 which is most important in discussions of 

 coastal subsidence. Below mean sea level the 

 salt-marsh builders, Spartina glabra and 8par- 

 tina patens, can not live. As a matter of 

 fact Spartina patens, by far the more impor- 

 tant of the two plants, seems to occur in sig- 

 nificant amount only within a vertical range 

 of about two feet. It builds its turf up to 

 mean high-tide level. Above this level it is 

 for the most part replaced by other plants, 

 notably by Juncus Gerardi and various spe- 

 cies of Scirpus. We may, therefore, regard 

 any thickness of Spartina patens turf greater 

 than two feet as a measure of change in high- 

 tide level. If such turf extends to mean sea 

 level, or below it, the evidence that subsidence 

 has taken place is indubitable. If a deep turf 

 is of uniform texture throughout, a strong 

 presumption is created that subsidence has 

 continued uniformly to the present time. 

 Spartina patens turf lies below mean sea level 

 at many places in the vicinity of Boston, as, 

 for example, near the mouth of the Saugus 

 Eiver, where it forms a homogeneous stratum 

 to a depth more than ten feet below high-tide 

 level. The geological significance of turf 

 formation by Spartina patens has been pointed 

 out by Professor C. A. Davis,^ of the Bureau 

 of Mines. He presents substantial evidence 

 that coastal subsidence is now going on — evi- 

 dence to which the hjrpothesis of a fluctuating 

 high-tide level has no possible application. 



Aside from the value of the salt marsh de- 

 posits themselves as indicating subsidence, 

 there is still another type of botanical evi- 



- " Salt Marsli Formation near Boston and its 

 Geological Significance," Economic Geology, V., 

 1910, p. 623. 



deuce which Professor Johnson should not so 

 lightly have disregarded. In certain cases 

 where fresh water peat is found below mean 

 sea level, in obviously undisturbed relation- 

 ship to the substratum, whether or not the 

 deposits have ever been invaded by the sea, 

 we have incontrovertible evidence of subsi- 

 dence. The full argument in support of this 

 statement has been published elsewhere,' but 

 may be briefly reviewed here. 



Peat deposits are of two main types, de- 

 pending upon the relation of the water table 

 to the ground surface in the depression in 

 which they are formed. In the first type the 

 depression contains a pond or other body of 

 water, so that the peat is derived from aquatic 

 plants and from the surface of the water, 

 through the agency of mat-forming plants. 

 If the water in such a depression were fresh, 

 a mat might be formed at or slightly above 

 sea level, and fresh-water peat of this type 

 would then be laid down below sea level. If 

 the ocean should break into such a bog it is 

 conceivable that the mat might settle consid- 

 erably and then be covered by salt-marsh 

 deposits. Such a condition I have never seen. 



The second type of peat deposit is built up 

 from the ground by successive elevations of 

 the water table, as we know from the char- 

 acter of the plant remains which it contains. 

 At every stage of growth its surface has very 

 nearly coincided with the ground water level. 

 Since in coastal Massachusetts the water table 

 is never lower than sea level, a bog of this 

 type, if its bottom is lower than mean sea 

 level, must of necessity be interpreted as a 

 record of subsidence. 



There is a locality at Quamquisset Harbor, 

 on the coast of Buzzard's Bay, where the sea 

 has cut into a Chamwcyparis bog. At the 

 water's edge a sounding showed uniform peat, 

 containing Chamwcyparis wood, to a depth of 

 fourteen feet below mean sea level. The sand 

 bottom of the deposit had not been disturbed 

 by under-cutting. At the point where this 

 sounding was made, there were salt-marsh 

 deposits in a thin layer overlying the peat, 



' " The Submarine Cliamseeyparis Bog at Woods 

 Hole, Mass.," Rhodora, XI., 1909, p. 221. 



