Crosby.] 496 [April 7, 
Mr. Atwood, so far as I am aware, stands alone among the stu- 
dents of this section in regarding it as composed chiefly of eruptive 
rocks. The radical difference between Mr. Atwood’s views and those 
of the other observers named is best shown by comparing the sec- 
tions published by him and Mr. Tate. These two sections cover pre- 
cisely the same ground, and yet, while Mr. Tate’s section consists 
almost entirely, at least nineteen-twentieths, of stratified rocks, these 
are found by actual measurement to form only one-sixth of Mr. At- 
wood’s section. | 
Mr. Atwood’s lithological determinations sre based chiefly upon 
notes (which are appended to his paper) of microscopical examina- 
tions of specimens by Prof. T. G. Bonney. But I venture to say that. 
his data here are very insufficient. In the first place only nine speci- 
mens were examined, and four of these—two gneisses, one horn- 
blende schist, and one quartzite — Prof. Bonney says are stratified. 
And of the remaining five specimens, only two— one basalt and one 
diabase, admitted to be eruptive by all observers — are set down pos- 
itively as eruptives. Of the other three Prof. Bonney speaks rather 
vaguely as follows:—“No.1. * * * * We may venture to say 
that the rock has once been a gabbro.” “No.7. * * * * The 
structure is rather abnormal; but I think it must be an igneous rock, 
and thus is a variety of quartz-diorite.” “No.8. * * * * The rock 
appears to be of igneous origin, and so is a microlinic syenite.” 
The lithological descriptions of LeNeve Foster, Tate, and Stevens 
are substantially the same; and from these we learn that between the 
Orinoco and Yuruari Rivers there is a great series of gneissoid rocks, 
with interstratified hornblendic and micaceous schists and quartzite. 
‘The gneisses predominate and are usually distinctly bedded and 
schistose, sometimes granitoid, micaceous as the rule, but hornblendic 
at several points, passing into diorite and syenite, in part highly gar-_ 
netiferous, and enclosing, in the Imataca Mountains, heavy beds of 
specular and magnetic iron ores. All observers are agreed that the 
iron ores are interstratified deposits. Portions of the gneiss also hold 
an abundance of endogenous quartz, and it is probable that some of 
the granitoid varieties are true granite. 
Comparison shows that this part of the section is essentially equiv- 
alent to the first and third divisions of Brown and Sawkins in British 
Guiana; while the east-west strike favors the view that the crystal- 
line formations of that region are continued to the westward as far, 
at least, as Caratal and the Caroni River. And this conclusion is 
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