144 NOTES OF A BOTANIST 
older than the horizontal sandstone of Erere and 
Santarem. - ^ 
The only other fact bearing on this point is in a 
recent letter from Prof. Branner. He says : " I was 
at Santarem and saw dark scoria-like rocks, prob- 
ably the same as those mentioned by Spruce. 
They closely resemble volcanic rocks, and were so 
compact that they broke with a glassy fracture ; 
but those I saw were sandstones cemented with 
iron and silica. Similar rocks occur about Para, 
and also over plains north and east of Macapa, 
where they cover large areas." These, however, 
I cannot think refer to the same rocks as those of 
the Serra of Irura and adjacent low hills, which 
correspond much better with those of the early 
Tertiary or Cretaceous formation described by Mr. 
Derby and Professor Hartt. The former writer 
states that throughout these beds " diorite is very 
common, forming immense dykes, and sometimes 
apparently forming sheets between the strata of 
sedimentary rocks." And again he says: ''The 
surface of these dykes is always decomposed, pre- 
senting a scoriaceous appearance, and enclosing 
crystals of quartz and fragments of the adjacent 
sedimentary rocks." Professor Hartt says that 
these dykes are often " so decomposed and eaten 
away that it is difficult to say what they originally 
were." 
These descriptive phrases will apply well to the 
scoriaceous rocks observed by Spruce and traced 
by him over so large an area, and I think they 
prove that on the north as well as on the south of 
the river both the newer Tertiary and the older 
Cretaceous rocks occur adjacent to each other, the 
