Ulrich on Correlation of the Lozver Silurian. 305 
Read the paragraph entitled "Surrounding Depths," (vol. i, p. 
237) together with the comparison in the next paragraph of 
the barrier reef of Australia to "a great submarine wall front- 
ing the sea," rising "at its seaward edge from depths which 
certainly exceed iSoo feet,"and farther on "such is the nature 
of these wonderful growths rising up in the midst of the great 
oceans," and see if the mental picture is not that of huge verti- 
cal walls standing up in the waters. That is the tone of all the 
text-books. Who would ever imagine, if he had access only to 
such descriptions, that the actual average slope is no more than 
45 degrees, and often as low as 30 degrees? Prestwich goes 
beyond Darwin or Dana in the sweeping inference of subsidence 
based upon the presence of coral formations. In his maj:), page 
235, he colors every reef region as a sinking area, no matter 
whether the formations are fringing reefs, barrier reefs, or 
atolls. 
We may conclude: 
1. That thin reefs, such as those of Florida, give no sujDport 
whatever to the theory of subsidence. 
2. That thick reefs may be formed either with or without 
subsidence. 
3. Atolls also may be formed on a stable as %vell as on a 
sinking platform. 
4. Deep lagoons, or steep slopes fronting the sea in deep 
water, cannot well be explained without subsidence. 
A CORRELATION OF THE LOWER SILURIAN HORIZONS 
OF TENNESSEE AND OF THE OHIO AND MISSISSIPPI 
VALLEYS WITH THOSE OF NEW YORK AND CANADA, 
nv E. o. ULRicir. 
{Couitmicdfrom f. igo.) 
A digest of the preceding list is as follows:' 
Total No. sp. mentioned from beds XI, XII, XIII, XIV. .450; restricted to same, S92 
•' XI 186; " " 108 
" XIa 183; " " 40 
' Certain errors in the range of the species given in the preceding 
table should be rectified. The species are referred to by their numbers. 
Thus, 49 is restricted to xri <?, instead of xii b\ 78 also in xii b\ add as 
