78 REVIEWS 
rock, and gabbros. Many of the volcanic eruptions took place under 
submarine conditions. ‘There were also periods of quiescence, during 
which fine sands and mud containing fossils were deposited. 
A subsidence of the sea floor ushered in the next period, the 
Llandeilo. The rocks of this formation are radiolarian cherts and 
mudstones which were deposited in clearer waters than the rocks of 
the Arenig. The rocks of the Carodoc are shales, conglomerates, and 
limestones, implying variable conditions of deposition. 
The Upper Silurian, it is said, is separated from the Lower Silurian 
by both physical and paleontological changes. But there appears to 
be no great paleontological break such as characterizes the separation 
of the Ordovician from the Silurian on the interior of the American 
continent. The transition from the Lower Silurian to the Upper 
Silurian types is a gradual one. This province may constitute one of 
harbors of refuge spoken of by Professor Chamberlin in his discus- 
sion of the source of provincial faunas. It would correspond, then. 
on the American continent, to the embayment at the mouth of the St. 
Lawrence. The following table will serve to compare the distribution 
of the Brachiopods common to the two countries, Scotland and 
America : 
Scotland America 
ae tena eppes n | Ordovician | Silurian 
NADA MATCHES oooccpecoue do bec CD00 x x x 
LNAI, WNANTTITAVIS. oo concdadoauocdassan x Be aK 
(Cyne, @xPOIMRSAI, o odoooocan00soec0 Co00 < x x 
Weptaenalrhomboidalisseyr elerteke itera % x xg x 
Plectambonites transversalis............ PK x ae x 
Blatystrophiaybitonatalyerrncicilericiereu x x x x 
Rafesinesquina alternata .............. x a x 
BIO MWS DUO 6400 s00G0s000000500000 x x a x 
Dalmanellagelesantulaeeiers peecrelere tnt x 58 a Ke 
Dalmanella testudinaria................ x x 
OU WOCSMATIE o 2556s co0ononooac0GKd0 xe ae x ae 
Pentamerusrobloncusie asec eee cece ce ae 5m x 
WUncinulussstricklandimeemrencinsrataele cereals XS Xe 
SpinifenicniSpusieeeee rie renter x x x 
‘Syomaliicie TEGMANENS cseodo mos oceogo ole eons x x 
Ratesinesquinasdeltoidearrme icin eet x x Ks 
Ratesinesquinmtagimllne xeaeertie niet ot-iponsnrye x a x 
Wola! OF GOSCISS 5 so0cndcoavcegesabe 14 II 7 12 
