mollusks

59:3.6The great vol­canic ac­tiv­ity of this age was in the Eu­ro­pean sec­tor. Not in mil­lions upon mil­lions of years had such vi­o­lent and ex­ten­sive vol­canic erup­tions oc­curred as now took place around the Med­i­ter­ra­nean trough and es­pe­cially in the neigh­bor­hood of the British Isles. This lava flow over the British Isles re­gion to­day ap­pears as al­ter­nate lay­ers of lava and rock 25,000 feet thick. These rocks were laid down by the in­ter­mit­tent lava flows which spread out over a shal­low sea bed, thus in­ter­spers­ing the rock de­posits, and all of this was sub­se­quently el­e­vated high above the sea. Vi­o­lent earth­quakes took place in north­ern Eu­rope, no­tably in Scot­land.

59:3.7The oceanic cli­mate re­mained mild and uni­form, and the warm seas bathed the shores of the po­lar lands. Bra­chio­pod and other ma­rine-life fos­sils may be found in these de­posits right up to the North Pole. Gas­tropods, bra­chiopods, sponges, and reef-mak­ing corals con­tin­ued to in­crease.

59:3.8The close of this epoch wit­nesses the sec­ond ad­vance of the Sil­urian seas with an­other com­min­gling of the wa­ters of the south­ern and north­ern oceans. The ceph­a­lop­ods dom­i­nate ma­rine life, while as­so­ci­ated forms of life pro­gres­sively de­velop and dif­fer­en­ti­ate.

59:3.9280,000,000 years ago the con­ti­nents had largely emerged from the sec­ond Sil­urian in­un­da­tion. The rock de­posits of this sub­mer­gence are known in North Amer­ica as Ni­a­gara lime­stone be­cause this is the stra­tum of rock over which Ni­a­gara Falls now flows. This layer of rock ex­tends from the east­ern moun­tains to the Mis­sis­sippi val­ley re­gion but not far­ther west ex­cept to the south. Sev­eral lay­ers ex­tend over Canada, por­tions of South Amer­ica, Aus­tralia, and most of Eu­rope, the av­er­age thick­ness of this Ni­a­gara se­ries be­ing about six hun­dred feet. Im­me­di­ately over­ly­ing the Ni­a­gara de­posit, in many re­gions may be found a col­lec­tion of con­glom­er­ate, shale, and rock salt. This is the ac­cu­mu­la­tion of sec­ondary sub­si­dences. This salt set­tled in great la­goons which were al­ter­nately opened up to the sea and then cut off so that evap­o­ra­tion oc­curred with de­po­si­tion of salt along with other mat­ter held in so­lu­tion. In some re­gions these rock salt beds are sev­enty feet thick.

59:3.10The cli­mate is even and mild, and ma­rine fos­sils are laid down in the arc­tic re­gions. But by the end of this epoch the seas are so ex­ces­sively salty that lit­tle life sur­vives.