WOODWORTH: GEOLOGICAL EXPEDITION TO BRAZIL AND CHILE. 995 
affording presumptive evidence that the sheet is an effusive flow. 
The road from Coritybanos northwestward to Sao Jodo traverses 
the deeply eroded surface of this sheet across the campos of Guarda 
Mor. In fact the upper waters of the Rio Marombas and its tribu- 
taries, the Rio das Pedras and the Correntes, mainly lie on this surface. 
South of Coritybanos there appears to be good reason for consider- 
ing the high trap mass there encountered as an outlier of a third sheet 
limited on the south by the Rio das Cachoeiras. This mass seems on 
account of the sandstones which crop out on its southern slope to be 
divisible into two beds of basalt. If this interpretation is correct, 
the top of the hill is a remnant of a fourth trap sheet in this area. 
Thickness of the Trap Sheets.— Exact estimates of the thickness of 
the several trap sheets are not possible from the notes I have; but the 
following approximations are thought worth recording as giving the 
order of magnitude of the thickness of the sheets. The estimate 
postulates the supposition that the base of the Corisco sheet is to be 
seen on the south bank of the Rio Correntes at an altitude of ca. 
3,400 feet, and that its upper surface passes beneath higher beds in 
the vicinity of Coritybanos at an elevation of ca. 3,200 feet. By 
graphic construction the sheet is found to be ca. 600 feet thick (190 M.). 
The overlying trap remnant south of Coritybanos consisting possibly 
of two flows must be nearly 600 feet (190 M.) thick with the inter- 
calated sedimentaries. The basal bed or Collectoria sheet on this 
estimate would be about 300 feet (95 M.) thick. The total thickness 
of the trap sheets would thus be about 1,500 feet (475 M.). 
Origin of the Trap Sheets — No known voleanic necks or explosive 
vents have been reported in connection with these sheets. In the 
region which I visited the succession of sheets is suggestive of surface 
flows and the Corisco sheet is very probably of that nature. Far to 
the southeast in Rio Grande do Sul, Dr. I. C. White has described the 
occurrence of sills and intrusive masses of trap as if breaking up 
through the sediments into the horizons occupied by the sheets in a 
manner to support the theory of fissure eruptions for the origin of the 
true flows of this region. In our journey across the sandstone tract 
north of Lages one noteworthy dike was encountered presenting 
characters which bear upon the mode of origin of the trap sheets. 
That occurrence will now be described. 
The Dike with Inclusions North of Lages—— About 8 kilometers north 
of Lages on the road to Coritybanos there is to be seen a remarkable 
dike agreeing in petrographic character with the neighboring trap 
sheets on the north and probably to be regarded as a feeder to one of 
