Diorite. 
291 
case with ice-borne erratics. The granite boulders 
are identical in composition with the surrounding 
rocks. 
Boulders of disintegration would be a fairly 
descriptive term for these rocks. 
It has been stated above that the granites form 
the lowest rocks in the geological series. That is, they 
underlie all stratified rocks, and if granite does rise 
into, and form mountains, even then it is the founda- 
tion of any sedimentary rocks in that particular 
locality. Reasons have already been given for believ- 
ing that the granite is a newer rock than the Silurian 
slate and Devonian Sandstones. The slates certainly 
rest on the granite, but the granite is, for all that, 
the newer of the two. 
Diorite. 
Like granite, diorite is a wholly crystalline rock. Its 
intimate structure is shown in Fig. 7 ; and the essen- 
tial minerals that go to make up the rock are felspar 
(plagioclase) and hornblende. Up to the present no 
diorite has been found intruded into the Hawkesbury 
Sandstones, near Sydney. Diorite dykes are common 
enough outside the sandstone area. It is thought for 
this reason that the diorites were erupted before the 
sandstones were formed. Diorite is often called “ green- 
stone,” a convenient term enough, provided we reserve 
it for diorites and allied rocks. There always seems a 
difficulty with beginners in distinguishing between 
diorite and basalt, although the distinction is not a 
matter of difficulty. The following tabulated state- 
