566 Scientific Proceedings, Royal Dublin Society. 



smooth flag, locally known as Dunmore slate. This, in old times,, 

 was used for roofing instead of slate, as will be seen on the old 

 houses in Dunmore, Tuam, and the neighbouring towns in the 

 Co. Mayo. It made a good substantial roof, the weight of the 

 " slates " being suitable to the heavy gales and storms of the 

 county. They were not very unsightly ; far less so than the 

 " Stourbridge slate," used in Oxford, England. They, however;, 

 required heavy timbering to support them. 



In the vicinity of Cong and Oughterard, the tracts of Lower 

 Carboniferous Sandstone are of limited extent, and the sandstone 

 is but little used on account of the excellent limestone in such ex- 

 tensive tracts in those localities. 



As loose stone in the islands, and along the shores of Lough 

 Corrib, are some peculiar sandstones. They have not been ob- 

 served in situ, and possibly may be of Silurian age ; but in 

 appearance they are more like the Carboniferous rocks. In 

 weathering, excrescences like small gooseberries grow out 

 from some, while others become pockmarked, small concave hol- 

 lows weathering into them. The latter stones, when weathered, 

 are extremely durable, as can be seen in the chancel arch of the 

 ancient churoh on Inchnagoill, in Lough Corrib. This arch was 

 restored some years ago by the late Sir B. L. Guinness, Bart., the 

 missing stones being supplied by ashlers cut from similar stones 

 picked up along the shore of the island. The old and new stones 

 were so similar, that now, after a lapse of thirty years, it is hard 

 to say which are the new ones. 



It is hard to explain the cause of the growing on the sur- 

 face of the stone of the " gooseberries." We learn, however, 

 from breaking a block that the " pockmarks " are due to small 

 globular secretions of ferrifero-chloritic matter, that rapidly de- 

 cay even when exposed to the air. After they are gone, the rest 

 of the stone is very durable. 



Sand and Gravel.- These in this county are interesting as 

 well as useful. In the low country, east of Galway Bay, and ex- 

 tending northward into the adjoining counties, are the Eskers that 

 are found more or less continuously aoross the central plain of Ire- 

 land ; and where they occur there is a plentiful supply of good 

 sand for building purposes, and also gravel for road metal. Out- 

 side the limits of the plain, good pit sand [can be obtained at 



