86 ROCKS USED IN THE MANUFACTURE OF TRONATTAS. 



In a paper previously read before this Society (i) I have 

 demonstrated that another, probably still more impor- 

 tant source where suitable substances were obtained 

 from are the gravel deposits of glacial and recent 

 age. It is needless to say that the pebbles contained in 

 these gravels are in a secondary position, and they must 

 have been derived from outcrops similar to those which 

 were used as quarries. 



If the metamorphic theory is correct, it stands to 

 reason that there must be a considerable number of 

 varieties of cherty rocks. A metamorphosed sandstone 

 must be considerably different from a rock whose origin 

 is a shale. But not only are the primary rocks widely 

 different in chemical composition, but each kind, 

 whether sandstone, shale, or anything else, varies con- 

 siderably. The cement which binds together the grains 

 of quartz in the sandstone may be calcareous or sili- 

 cious, and, above all, the percentage of iron varies con- 

 siderably in each class of rock. It is therefore a priori 

 very probable that a large number of varieties as to 

 colour and other physical qualities must be the result 

 of metamorphism, and I wish to deal in this paper with 

 the main types that can be distinguished. 



Whenever a collection of native stone implements is 

 made in the island, and such collection be sorted after- 

 wards, it will be found that two large groups can be 

 distinguis'hed at once, viz. — 



1. Volcanic rocks, 



2. Metamorphosed sedimentary rock of a highly 



siliceous nature. 



Though I propose to deal here only with the siliceous 

 rocks, it will be advisable to say also a few words about 

 the volcanic rocks. One fact becomes conspicuous at 

 once — not a single chipped implement has been found 

 that is manufactured from a volcanic rock. All the 

 chipped implements, the tronattas s.s.; are invariably 

 manufactured from the siliceous rocks. 



Not in a single instance has this rule been broken, 

 and the reason for this is, as we will see later on, quite 



(i) Notes on a Chipped Boulder found near Kempton, 

 " Pap. and Proceed. Roy. Soc, 1908." 



