126 OEIGIN AND DEVELOPMENT OF ORE- DEPOSITS. 



Carcla%e. This famous open-work is two miles northward 

 from St. Austell. It is now worked almost exclusively for china 

 clay, but was formerly worked for tin only ; its records, it is said> 

 date from the time of Henry YII. The most recent published 

 description of Oarclaza is that given by Mr. E. Symons in our 

 Journal. Mr. Symons found the area to be, by actual survey, 13 

 acres, and the greatest depth 132 feet.* Tbis shows a very great 

 increase from the 6 acres surveyed by Mr. Thomas, previous to 

 1816, f but the extension has been almost entirely in the "clay- 

 beds" to the northward, and scarcely at all on the schorlaceous 

 and stanniferous branches. At the time of Mr. Thomas's visit in 

 Jan., 1830, there were 8 stamping mills at work, shafts had been 

 sunk 10 fathoms deeper than the bottom of the pit, and the mine 

 was said to be rich in the bottom. The clay was merely refuse, 

 to be washed out through the adit as speedily as possible. He 

 calculated that one million tons of stuff had been thus removed. 



The schorlaceous tin branches run nearly E.W. and parallel 

 to the junction with the killas (tourmaline schist); they vary from 

 a fraction of an inch up to 2 feet in thickness, the thicker ones 

 being almost devoid of tin. The greatest length of the workings, 

 including the eastern part known as Little Carclaze, is nearly 

 half-a-mile, and the total quantity of tin-bearing ground removed 

 must be at least one million tons, besides several million tons 

 of non- stanniferous clay ground. The pit must now (1892) be 

 at least 18 acres in extent at the top. 



Rock Hill. The abundance of schorl in connexion with the 

 tin at Carclaze, is still more noticeable at Eock Hill. This hill 

 is situated to the left of the turnpike road from St. Austell, and 

 about half-a-mile short of the village of Bugle. A number of 

 tin lodes formerly worked with considerable advantage in the 

 Eocks mine, just under the turnpike road, converge at Eock 

 Hill, where they have been worked at intervals for generations 

 in an open quarry. Very little has been done there for the past 

 few years, so that my description of the place, published with 

 sketches, in 1873, needs but little alteration now. It runs as 

 follows: — ''The main excavation is of a nearly circular form, 

 not much less than 150 feet diameter, and about 40 feet deep. 



* Journ. Eoy. Inst. Corn., ix, p. 140, 1877. 



f Henwood, Trans. Eoy. Geol. Soc, Corn., v, p. 120. 



