§7 

 THE OCCURRENCE OF THE 



RARE MINERAL, MONAZITE, IN THE MILL- 

 STONE GRIT OF YORKSHIRE. 



A. GILLIGAN, B.Sc, F.G.S. 



Leeds. 



The presence of fragments of garnets in the Millstone Grit 

 was recorded by W. C. Trevelyn as far back as 1835, and other 

 workers have since detected them in the same series of rocks 

 in widely separated districts. They have been found by the 

 writer to be very common in all the coarser beds of the series 

 examined by him. Generally, they are sporadically scattered 

 through the mass of the rock, but a fortunate discovery of 

 garnet if erous layers in the Rough Rock of Cragg Hill Quarries, 

 Horsforth, called for a closer investigation, not only of the 

 garnets, but of the associated heavy minerals in these layers. 

 These include zircon, tourmaline, rutile and monazite. This 

 last mineral, which is a phosphate of the rare earths cerium, 

 lanthanum and didymium (Ce, La, Di) PO-4, with some silicate 

 of thorium, Th Si O4, occurs as rounded, honey-yellow grains, 

 which, however, are often clouded by alteration products. 

 No trace of crystal outline has so far been observed, although 

 the zircons with which it is associated, show perfect crystal 

 forms, the faces being easily determinable. This difference 

 in form is, of course, accounted for by the lower degree of 

 hardness of the monazite, which is only 5.5 (Moh's scale), as 

 compared with 7.5 of zircon. 



Though only a small quantity of the monazite has so far 

 been separated, it has yielded satisfactory chemical tests for 

 the phosphate. The most reliable test however, is afforded 

 by the spectroscope, the mineral when examined by direct 

 sunlight giving an absorption spectrum which makes it un- 

 mistakable. Prof. Bowman, of Oxford, has very kindly 

 confirmed the spectroscopic tests, so that there can be no doubt 

 of the correct identification of the mineral. 



Since monazite occurs in granites and granite gneisses, and 

 especially in crystals of considerable size, in pegmatites, its 

 presence in such a rock as the Millstone Grit is not surprising, 

 for the lithological character of the grit makes it quite certain 

 that it was derived from such types of rocks as those mentioned 

 above. 



Having with certainty determined its presence in the Rough 

 Rock, a search was made for it in other beds of the Series, and 

 it is found to be of widespread occurrence. It has been found 

 in the Kinderscout Grit and Middle Grits of several localities. 

 It occurred to the writer that it might also be found in the 

 glacial sands of Airedale, such as those at Newlay, and on 

 examination, this was found to be the case. In these sands 

 it is associated with the usual heavy minerals of the Millstone 



1917 Mar. 1. 



