362 Scientific Proceedings, Royal Dublin Society. 



Part II. 

 The highly Magnetic Nickelij-errous Serpentine, from Slishwood, 



near Sligo. 

 This rock is of a rare character, and I believe that up to the 

 present no serpentine containing magnetite in quantity has been 

 recorded as having been noticed in Ireland. So far as I am aware 

 the only instances given of such a rock are those cases mentioned 

 by Bischof, as occurring at the Heidelberg, near Darmstadt ; and 

 at the Auschkul Lake.* The description of the first of these 

 resembles in the closest manner that of the Sligo rock, as will be 

 seen hereafter. 



The serpentine at Sligo forms a well defined and very regular 

 band amongst the metamorphic rocks on the southern shore of 

 Lough Gill, and extends from Bunowen Bay along by Slishwood, 

 in a south-easterly direction for about a mile and a half, when 

 the Carboniferous Limestone supervenes, and cuts it off. It has 

 a thickness of about 500 feet, being, as observed by Professor 

 Hull, the most extensive and regular band of serpentine in 

 Ireland. The section on page 359 will explain the manner in 

 which it occurs. 



Physical Characteristics. — The rock is apparently an ordinary 

 common serpentine — dark coloured, compact, and somewhat hard. 

 The bedding is very distinct, and along the planes of bedding, 

 well defined bands may be seen, which, at first sight, appear to 

 be bands of grit, but on closer inspection prove to be bands of a 

 hydrous silicate in all essential points resembling serpentine, but 

 containing a larger quantity of water. In all parts of the rock 

 mao-netite occurs, but chiefly on the western side of the band. To 

 the eastwards the rock is more homogeneous, and it was when ex- 

 amining the tiny black particles which appear in this part of the 

 rock, that I became aware of the magnetic character of it. In 

 the most compact specimen of this rock minute grains of 

 black matter will be seen, and on powdering a portion, and run- 

 ning a magnet through it, a quantity of magnetite can be 

 extracted. Tracing the rock to the westwards, this mineral 

 becomes more and more abundant, and close to the western 

 boundary occurs in very large quantity indeed, and portions of 

 the rock possess all the characters of natural magnets. They 

 attract and repel the magnetic needle, and have distinct polarity. 



* See chapter on Serpentine. Chem. Geol., Biscliof, 



