1849.] LYELL ON THE STRUCTURE OF VOLCANOS. 211 



the Purbeck and Portland strata prevents the waves and tides from 

 breaking down and widening the seaward barrier, and the compara- 

 tive softness of the vertical or highly inclined beds between the bar- 

 rier and the chalk at the head of the bay promotes the enlargement 

 inside the entrance. 



4thly. But there are certain valleys in Australia, described by 

 Mr. Darwin, which from their depth, the steepness of their boundary 

 cliffs, and the narrow gorges by which the sea has entered to hollow 

 them out, afford perhaps a still more striking explanation of the mode 

 of operation, to which I shall refer in great part the origin of such 

 craters as Palma, the Gulf of Santorin, and others of similar large di- 

 mensions. I allude to those valleys near Sydney in New South Wales, 

 by which the great platform of sandstone, 1200 feet thick, is pene- 

 trated. The traveller, says Mr. Darwin, when walking over the sum- 

 mit plains, finds himself suddenly at the brink of a continuous line 

 of lofty cliffs, so perpendicular, that he can strike with a stone the 

 trees growing at the depth of between 1 000 and 1500 feet below him. 

 At the distance of several miles he beholds the opposite line of cliff, 

 rising up to the same height with that on which he stands, and 

 formed of the same horizontal strata of sandstone. So continuous 

 are the bounding lines of cliff, that to descend into some of these 

 valleys it is necessary to go round twenty miles ; but what is still 

 more remarkable, these valleys, although several miles wide in their 

 upper parts, generally contract towards their mouths to a mere 

 chasm, impassable to man or beast. Thus the gorge of the Cox 

 river is only 2200 yards wide, and about 1000 feet in depth. Mr. 

 Darwin at first asked himself whether the mass of stone removed 

 from these great amphitheatrical depressions, had not subsided ver- 

 tically ; but was compelled to abandon this notion on considering the 

 narrow promontories which projected from the platforms into the 

 valley. He was also struck with the resemblance of the inland ba- 

 sins or bay-like recesses to the present bold sea-coast, where there 

 are similar recesses forming fine harbours, connected with the sea by 

 narrow mouths, sometimes not more than a quarter of a mile in width, 

 the cliffs being formed of similar sandstone. 



5thly. It must also be remembered that in the coasts of volcanic 

 islands, such as Palma, Santorin, St. Helena, and others, there are 

 lofty cliffs of basaltic and other igneous rocks, often traversed by dikes 

 which have been formed by the undermining action of the sea, and 

 are still wasting away. Mr. Darwin has particularly dwelt on the 

 enormous cubic mass of hard rock pared off by the swell of the At- 

 lantic from the coasts of St. Helena, where there are perpendicular 

 cliffs from 1 000 to 2000 feet in height, consisting of basaltic strata 

 traversed by dikes (p. 91). In this island also, as well as in St. Jago 

 and Mauritius, he has observed in his volume on Volcanic Islands, 

 p. 93, that the ring of basaltic mountains forming what is commonly 

 called '*the crater of elevation," must once have been nearly or quite 

 continuous, although now broken. Some very wide breaches have, 

 he observes, been evidently effected by the denuding action of the 

 waves. All these islands, he concludes, have been elevated in mass. 



