Geology and Mineralogy. 327 
structure, which is, however, probably the same as that of the 
Itaparica re reef, 
enter very largely into its — pro s great an 
extent as the nullipores, and giv e to an exceedingly hard, 
calcareous rock from which, nltinitasuiiy: all traces of the’ worm: 
tube crc ta: e disappear. e worm-tubes and nullipores evi- 
whi t 
growths of eohiiilin pies arge digitate nullipores, so common 
at Pernambuco and at ae places in the Bay of Bahia, are 
limited to the lower part of the reef, where they are associated 
with the true corals. At present nullipores are living in abun- 
dance only on the outer side of the reef, to a height of about one 
foot above medium Jow tide. Above the line of nullipores we 
find the entire upper surface of the reef coated with a layer of 
living worm tubes and large barnacles. The latter are generally 
but the former remain, pro- 
compact below. The existence of nullipores in this upper portion 
indicates, however, that they lived on top of the reef at no dis- 
tant time, and probably also that the reef has been elevated to a 
slight extent since then. 
br the reef the water is very opeeee being deepest near 
the reef and especially at and around the openings through it; 
it seradiinlly shallows inward toward the bewn The bottom of 
this shallow inner channel is covered with sand and fragments of 
C 
agments are all old, seo oes much worn, ee set 
ozoan growths, also 
se 0 eds of considerable thickness in places, often 
more or less consolidated, and are dug up to burn for lime. The 
species discovered among the Bee taba 8s are all found living in 
various parts of the bay, excepting : d 
hot apparently live at present wepaban in the Bay of Bahia. 
T nsive accumulation of broken corals, which must have 
that s have Seen. ceased to be reef- builders i in 
the eBay at Bah 
Bee ew Sossiliferous rocks containin 
—Dr, . Dawson mentions facts on this subject from which the 
following are cit 
- Jour. Scr, Tare Serres, Vou. XVII, No. 100.—Aprit, 1879, 
