Vol. 4] Lawson. — The Robinson Mining District. 



299 



of the Basin Ranges, and which have generally been referred 

 to the Archaean. In this case the mass is clearly of post-Car- 

 boniferous age and there is not wanting evidence that this will 

 be found to be true of many other similar rock masses. The 

 monzonite appears in the Rib Hill anticline and underlies the 

 relatively low and gently sloping ground of Weary Flat in the 

 western part of the district. Its occurrence is marked by no 

 prominent outcrops nor even by hilly ground. The area occu- 

 pied by it can be delimited with a fair degree of precision. The 

 regolith which thinly veneers it is so characteristic of the under- 

 lying rock, that, taken with the occasional exposures of the latter, 

 and the opportunities afforded by the rather numerous prospect 

 holes that have been sunk upon it, particularly about its peri- 

 phery, there has been no serious difficulty in mapping it. The 

 longest diameter of the area is about one mile in a northeast- 

 southwest direction and its width is about half a mile. 



The rock composing the mass is in general a rather coarse 

 grained, pinkish to gray hornblende-monzonite, with large por- 

 phyritic crystals of orthoclase, and has a characteristic plutonic 

 habit. When this rock was first met with in a rapid survey of 

 the field to be studied, the writer, relying upon previously re- 

 ported occurrences of Archaean granitic rocks in the basin 

 ranges, naturally supposed that he had here to deal with the 

 basement upon which the Palaeozoic rocks were laid down. In 

 testing this hypothesis, however, several facts came to light which 

 seemed to be inconsistent with it. It was remarked with surprise 

 that the lower part of the Palaeozoic column, including the Silu- 

 rian and Cambrian which form voluminous and persistent ter- 

 ranes throughout this part of Nevada, was here absent. The 

 lowest rocks in contact with the monzonite are those referred 

 in this paper to the Devonian. In meeting this objection it was 

 considered barely possible that a pre-Devonian disturbance and 

 erosion interval had removed the earlier Palaeozoic rocks, or 

 that this portion of the region had remained an insular mass in 

 Cambrian and Silurian times and had been finally submerged 

 only with the advent of Devonian time, and that the monzonite 

 might thus still be regarded as a portion of the Archaean floor 

 upon which the Palaeozoic in general was laid down. This possi- 



