1 2 Consolidation of Strata. 



is also as a whole one of elevation. The same is true 

 of the volcanic islands of the Pacific, and also of Java, 

 which contains many active volcanoes, and around the 

 shores of which there are old coral reefs 140 feet above 

 the level of the sea. Under other circumstances a 

 great number of coral reefs of the kind called atolls 

 and barrier reefs, yield, according to Darwin, perfect 

 evidence of depression of land. In the Pacific an area 

 more than 4,000 miles in length is now undergoing this 

 kind of submergence. The same takes place in the 

 Laccadive and Maldive archipelagos in the Indian Ocean. 

 All these islands are non-volcanic. Where volcanoes 

 occur the land is generally rising. 



During such depressions strata may accumulate to 

 an immense thickness under favourable conditions of 

 supply, and time being also allowed for consolidation, 

 when these are again unheaved they will, both as regards 

 quantity and structure, be more apt to resist destruction 

 than smaller masses of (probably) softer strata that were 

 formed during periods of minor oscillations of sea and 

 land. 



Strata are consolidated (petrified) chiefly by pres- 

 sure and chemical decomposition and recomposition. 

 Some formations are many thousands of feet in thick- 

 ness. In a set of strata 10,000 feet thick, the super- 

 incumbent weight on the lowest bed would be about 

 12,333 Ibs. per square inch; but beside this, more 

 intense pressures have taken place throughout all but the 

 very latest geological epochs. This kind of pressure has 

 been brought about by contraction of the crust of 

 the earth due to radiation of the proper heat of our 

 globe into space, the result being, that over broad areas 

 rocky masses have been much contorted and compressed, 

 and thus mountain ranges have been upheaved. In some 



