The Shinumo Area. 503 



is a thin conglomerate. This constitutes the basal member of 

 the Unkar. Overlying the conglomerate is a series of lime- 

 stones and calcareous shales. These grade upward into argil- 

 laceous and arenaceous shales which are intruded by a thick 

 sill of diabase, and are succeeded in turn by great thicknesses 

 of sandstone and quartzite. The uppermost exposed member 

 of the group in the area is a thick series of micaceous shaly 

 sandstones. 



It has been shown that these strata lie in a wedge-shaped 

 mass inset in the Vishnu schists, and that this wedge is com- 

 posed of a great number of smaller titled fault blocks. It is 

 apparent from this relation that nowhere in the Shinumo area 

 can the thickness be measured in one unbroken section. Since, 

 however, the lithological characters of the strata are constant 

 and easily recognized, and since the throw of the faults that 

 bound the titled blocks seldom exceeds 100 feet, the restoration 

 of a section showing the unbroken sequence is not a matter of 

 great difficulty. 



Detailed sections were made upward from the base of the 

 Unkar through each fault block until its limiting fault was 

 reached. The highest bed measured was then located in the 

 next block to the northeast, and the measurement resumed at 

 that point. Except in the fifth, or highest member of the 

 group, all sections were measured with a tape along the nearly 

 vertical wall faces of the box canyons of the Shinumo and 

 other washes that cut across the strike of the strata. In the 

 fifth, or highest member of the group, the strong drag of the 

 great fault on the northeast has flexed and contorted these 

 shaly sandstones in such a manner that accurate measurement 

 with the tape alone was impossible. Their thickness was 

 computed by the aid of trigonometric formulae, using the com- 

 bined data afforded by the use of the tape, the topographic 

 map, and observations of the varying strike and dip. 



The section incorporated in this article was made in two 

 places. The greater part of the total thickness was measured 

 in a traverse up the Shinumo. This section includes all the 

 strata above the diabase sill which is intruded midway in the 

 " arenaceous and argillaceous shales " which comprise the third 

 member of the Unkar. It would have been perfectly possible 

 to make a complete section of the group in a traverse of the 

 entire course of the Shinumo from the basal unconformity at 

 the mouth of the creek to the great fault three miles above, 

 although four faults cross the creek between its mouth and the 

 place where the diabase sill dips beneath the bed of the stream. 

 But a place was found in the canyon of the East Wash where 

 all the strata between the basal unconformity and the diabase 

 sill lie in a continuous unfaulted section, in a fault block that 



