202 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The step down from this red-rimmed plateau to the Aubrey platform is 
broken by an intermediate platform and big flat topped mesas of gayly 
colored shales capped by a resistant conglomerate bed. Beyond them 
stretches the vast Aubrey platform. Thus, as one looks eastward to- 
wards the plateau country, the impression is uniform horizontality. 
Turning about towards the west, one sees a different country. Instead 
of the flat plateaus there are low hills of irregular form, and mountain 
ranges beyond. ‘To the northeast rise the smooth slopes of the Pine 
Valley mountains, generally subdued in form though already somewhat 
scarred by recently revived activity of erosion. Between the range and 
Toquer hill lie foot-hills, whose irregularity in arrangement of colors 
indicates a rock structure complicated by folding, in contrast to the 
regularity of color so noticeable in the plateaus to the east. Towards the 
southwest the view is limited again by the hills about St. George. In 
the foreground a lowland stretches from north to south, along the base 
of the Hurricane-Bellevue cliff front. Near this escarpment the lowland 
is generally flat and is partly buried under lavas, but here and there 
black basaltic cones show the sources from which these lavas spread out. 
Northwest of Toquerville two broad alluvial fans stretch out from the 
Pine Valley range over the lowland, and these, with the lava sheets, 
conceal the rock structure over a considerable area. 
Such, in outline, is the appearance of the district in which we worked. 
Our main interest lay in the history of the Hurricane fault, and its 
bearing upon the broader history of the whole plateau region on its 
eastern side. Of equal interest was the study of its neighbor, the Grand 
Wash fault, and it is to the evidence along these two fault lines that we 
wish to direct especial attention. It seems wise, however, before taking 
up the main theme — the Tertiary history of the region — to consider 
briefly the earlier history, chiefly Mesozoic. 
The Rock Series. 
The question of the subdivision of the rock series into formations has 
always been puzzling. Newberry, Powell, Gilbert, and Howell contrib- 
uted much towards an understanding of the stratigraphy of the plateau 
region; but the scarcity of characteristic fossils made it impossible to 
divide the series satisfactorily into formations on a paleontologic basis. 
In his elaboration of the work of these men, however, Dutton has 
sought, both by direct fossil evidence and by rather doubtful correlations 
with strata in neighboring provinces, to identify several formations as 
