OPHITES AND RELATED ROCKS. 27 



glacial drift has been broken up, and its finest materials, clay 

 and sand, washed off leaving a mass of pebbles, cobbles, and 

 large erratics, consisting of granite, syenite, and other rocks, 

 piled up into an esker. This deposit has lately yielded to our 

 examination several pebbles and blocks of syenite, in which 

 are veinlets and isolated patches of calcite, obviously the 

 result of changes effected in the component hornblende and 

 oligoclase, these minerals, where the calcite is present, being 

 more or less corroded, and having a spongy structure. Minute 

 patches of serpentine associated with the calcite are also occa- 

 sionally seen. When the calcite has to some extent been 

 removed by dilute acid, there are generally left siliceous bodies 

 in the form of rude arborescent configurations rising out of 

 the remaining calcitic matrix, in one instance strikingly like 

 those that are common in the cases elsewhere mentioned. 

 Later investigations on the massive diorite, which has been ex- 

 cavated during the past year (1880) in the construction of the 

 new dock in Galway harbour, have yielded us a number of spe- 

 cimens containing calcite in abundance, the secondary origin of 

 which is indisputable. 



We have also found very recently in situ, near Salt Hill, Gal- 

 way, a porphyritic feldsyte more or less serpentinous, at a spot 

 where a quarry has been opened in a fruitless search for copper- 

 ore. The serpentine is generally seen lining fissures, often 

 superficially but occasionally to a depth of a few inches : it also 

 occurs in detached pieces of rock lying about. A loose block 

 a cubic foot in size, and altogether serpentinous, was taken out 

 of an adjacent wall. The mass greyish, brownish, and olive- 

 green in colour, has in some parts quite a soapy feel, an oily 

 lustre, and a coarse fibrous or slickenside-like structure, resem- 

 bling in these respects baltimorite and pyrosclerite. 



These are examples of changes effected by chemical reactions 

 in igneous rocks, certain of whose original mineral substances 

 having undergone replacement by serpentine and calcite *. 



We are now, however, trenching on chemical changes that 

 have taken place in rock-masses a subject which properly 

 belongs to the next Chapter. 



* Specimens of the examples referred to ore deposited in the geological 

 museum of the Queen's College, Galway. 



