132 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
found faults” (a, 8), and leaves the erosion that they have suffered to be 
inferred. Elsewhere, when describing the West Humboldt range, he says: 
“The precipitous mountain face . . . is inreality an ancient fault scarp 
of grand proportions, which was somewhat eroded before the existence of 
Lake Lahontan” (a, 277) ; but “somewhat eroded” does scanty justice to 
the fine sculpturing of this range as shown in the accompanying Plate 
XLV. Spurr distinguishes between scarps directly due to faulting, and 
scarps due to the erosion of a long-ago faulted mass ; but he gives no ex-_ 
plicit discussion of the forms assumed by a simple fault scarp as it under- 
goes dissection ; and his attention to the physiographic features of the 
Basin ranges in general is so brief that he implies that they possess an 
intimate correlation of structure and form by saying that the Appa- 
lachians “ likewise consist of parallel ridges eroded along lines of fold- 
ing” (255). 
In spite therefore of the many descriptions of the Basin ranges that 
have been published, there has not yet appeared any detailed statement 
of the theory by which they are explained ; the essential consequences 
of the theory have not been explicitly formulated ; the criteria by which 
a fault-block mountain may be recognized in early or later stages of dis- 
section have not been defined ; and it is to supply these deficiencies that 
the preparation of this essay was undertaken. 
When the essay had reached an almost completed form, the writer 
had the advantage of hearing the Basin range problem discussed by Mr. 
Gilbert at the Washington meeting of the Geological Society of America, 
in January, 1903. It was a gratification to find that the plan of presen- 
tation here adopted very closely resembled in various ways the treatment 
offered by the originator of the Basin range theory: it was at the same 
time an embarrassment to see that many of these pages would be hardly 
more than repetitions of Mr. Gilbert’s report. They may, however, have 
a certain value in so far as they show that independent study leads to 
accordant results. 
Ideal Types of Fault-block Mountains. — There are two chief types of 
fault-block mountains as illustrated in Figures 1 and 2: one shows what 
may be called a tilted block, the other a lifted block. In order to econ- 
omize space, only the tilted block type will be here considered in detail. 
The most characteristic features of a typical tilted block mountain in 
its youth or early maturity may be summarized in Figure 1 in which the 
block, acr, has been raised and more or less inclined. The upper part, 
Bc, of the faulted face, ac, rises above a piedmont plain of waste, BD, 
by which the backward slope of an adjoining block is buried ; while the 
