ST. MARTIN ROCKS — THUNDER ISLAND. 



33 



St. Martin Eocks. It is not improbable that the epoch 

 of their elevation was simultaneous with outbursts which 

 have been observed in other parts of the continent. At 

 noon we arrived at a semicircular island of beaches 

 similar to those at the east end of the lake. They are 

 due to the great shallowness of St. Martin Lake, which, 

 with an area of over three hundred square miles, was in 

 no part of our course found to be more than eighteen 

 feet deep, and often only five and six feet, for long dis- 

 tances. 



In the afternoon we landed on an island where 

 stratified limestone, in horizontal layers, was exposed. 

 The limestone possessed some singular peculiarities. 

 Numerous cup-shaped forms, of very large dimensions, 

 were visible in projecting masses over the whole of the 

 surface exposed. Many of these cups were fully thirteen 

 inches in diameter at the surface, and would hold at least 

 one quart of water. They consisted of concentric rings, 

 or cups, regularly arranged, and from ten to fifty or more 

 in number. The thickness of each cup varied from one- 

 tenth to one-quarter of an inch. A single specimen 

 resembled a gigantic onion which had been cut in half, 

 with a few of the inner layers extracted, leaving a cavity 

 or depression. Many square yards of surface were varie- 

 gated with this structure. The colour of the limestone 

 is a buff-yellow, its fracture is uneven, and masses are 

 difficult to separate, being extremely hard and siliceous. 

 The height of the exposure is sixteen feet, and so nearly 

 horizontal, that no inclination could be detected. The 

 island having no name, and being remarkable for its rock 

 formation, it was thought worthy of some designation : 

 we therefore called it " Thunder Island," in memory of a 

 storm of hail and rain, accompanied by lightning and 

 thunder of more than ordinary violence, which made us 



VOL. II. d 



