472 PHYSICAL GEOGRAPHY. 



material was volcanic, and, as these islands rose higher and higher, the 

 space between them and the coast cliffs also rose until it became dry 

 land. Soon rain fell and accumulated in this valley so formed, and lakes 

 and rivers appeared. In these, diatomacerc appeared, thrived, grew, 

 reproduced, and multiplied ; lacustrine sedimentary deposits were thrown 

 down. Now came a time when the volcanic cones, which constituted the 

 peaks of the range of mountains nearest to the coast, burst forth with fire 

 and lava ; and, probably at the same time, earthquakes took place which 

 drained many of the lakes and changed the courses of rivers. Into the 

 lake basins the lava was poured, with its heat evaporating the moisture, 

 and consolidating the diatomaceous material into a stony mass, from 

 which all organic matter was burned out. A period of rest succeeded. 

 Diatoms again appeared and accumulated, to be again overlaid by lava; 

 and so on the same thing may have again and again taken place. In 

 this way the enormous deposits of sub-plutonic diatomacerc were formed ; 

 and in the cracks, made in the rock by volcanic agency, the rivers 

 wended their way, and made the gates we now are in the habit of call- 

 ing canons. 



But in the ocean diatomacex also occur, and in large quantities. 

 When they die there, their siliceous remains must accumulate at the 

 bottom of the water, and occur as deposits. It is in the black mud of 

 our quiet bays and harbors that we must look for the greatest accumu- 

 lation of these remains ; and rivers arc carrying them down to their 

 mouths, where often they are piled up in such masses as to form bars. 

 The mud of the river Thames in England yielded to Mr. Roper a large 

 number of diatomaceous remains. Ehrcnberg examined the mud of the 

 Elbe in Germany, and found these minute shells to make up from one 

 quarter to one third of the whole mass. lie calculated that at Pillau 

 there arc annually deposited from the water from seven thousand two 

 hundred to fourteen thousand cubic metres of these minute shells, 

 which in the course of a century would give a deposit of from seven 

 hundred and twenty thousand to one million four hundred thousand 

 cubic metres of deposit, which might be hardened into a stony mass. 

 That such hardening has taken place is evidenced by the occurrence of 

 the vast strata of marine forms found in Virginia and Maryland, on the 

 Atlantic side of North America, and in California on the Pacific coast. 



