190 Correspondence — H. T. Ferrar. 



Fleming rocks are very important, both on shore and at sea. The 

 islets of the Mewstone, Eastern Blackstone, and Western Blackstone 

 are all volcanic. The Mewstone, with its southern mass of diabase 

 abutting on its northern vertical slates, is as like the green ' schist ' and 

 mica-schist at the Start as a raw egg is to a boiled one ; and I fully 

 believe their relations are very similar thereto. A. E. Hunt. 



CAVITIES IN CRYSTALLINE ROCKS. 



SiR^ — I notice in the February number of the Geological 

 Magazine a letter from Professor Bonney on the subject of atmo- 

 spherically eroded rocks. In this connection it may be of interest to 

 put on record the fact that hollowed rocks, apparently quite like 

 those described by Mr. Tuckett, Professor Bonney, and the Rev. K. 

 Baron, were met with under totally different atmospheric conditions 

 in the Antarctic. The examples there also occur in granite. They 

 are found at an altitude of about 4,000 feet in latitude 77° 49' S., 

 longitude 163° E., in South Victoria Land, and at least two types 

 may be distinguished. 



A. In fairly normal granite. The rock is a very ordinary grey 

 to pink granite with felspars usually about a quarter of an inch long ; 

 it appears to be quite fresh even on the surface, and has a marked 

 superficial glaze on both convex and concave surfaces. The most 

 striking cavity is on the south and weather side of a large block, and 

 therefore faces away from the sun; it is about eighteen inches across 



Fig. 1. — A large block of granite showing ca\dty on the south and -weather side; 

 1 8 inches across at opening, diameter increasing to 2 feet inwards ; depth of 

 cavity more than a foot. 



At the opening, and the diameter increases inwards to at least 

 two feet. The depth of the cavity is a little more than a foot, and 

 the back wall is partially covered with a hard mammillated or 

 botryoidal crust, the surface of which is white and harsh to the 

 touch. Pieces of this were brought home, and some of these 

 Mr. Prior has kindly analysed for me: he says, "the incrustation 

 consists mainly of carbonate of lime; there is a little silica left 

 behind on solution in hydrochloric acid." The incrustation was 

 lamellar, scarcely more than one-eighth of an inch thick on the 

 average, but in the projecting botryoids, which are sometimes 

 partially hollow, may be more. The incrustation was firmly fixed 

 to the granite face, and it was impossible to make out whether the 

 surface beneath it was or was not glazed. 



B. In a very coarse granite with abundant large crystals of 

 orthoclase. The hollowed blocks are rounded, but owing to the 



