﻿138 CD. Walcott — Cambrian System of North America. 



in the California gold. A beautiful specimen in the cabinet of 

 Professor Brush from the Spanish Dry Diggings in El Dorado 

 county shows it very plainly. This specimen consists of large 

 crystalline plates bound together by a little quartz ; the plates 

 are in part solid with triangular markings on the surface, or 

 hexagonal depressions made by the planes of this hexoctahe- 

 dron, and in part consist of open work formed of delicate crys- 

 talline ribs branching at angles of 60° and 120°. These plates 

 are essentially flattened octahedrons, and their edges are all 

 formed by an oscillatory combination of the hexoctahedral 

 planes, producing a fine striation similar to that on the speci- 

 men before described. Occasionally the edge is beaded with 

 distinct crystals, and in cavities other crystals are to be seen, 

 the small ones entire, or at most, with hexagonal pittings (figure 

 8), the larger ones, skeleton forms, as if formed of gold wire. 

 Measurement proved the hexoctahedron to be the same as that 

 already determined, while a second hexoctahedron t, 4-2 (421) 

 is sometimes sufficiently developed to admit of determination. 

 The writer has recently had an opportunity to examine the 

 fine series of crystallized gold in the cabinet of Mr. C. S. Bement 

 of Philadelphia, and has observed a number of specimens show- 

 ing the hexoctahedron 18-f and very similar in habit to those 

 described. Two of these specimens were from El Dorado 

 county ; another from Tuolumne county was octahedral in 

 habit with the edges replaced in the manner described by the 

 oscillations of x ; still another from Yreka county showed 

 several thin triangular plates with the striations on the edges, 

 and with also hexagonal pittings formed by the planes of the 

 same hexoctahedron. In the article by Prof. Blake, already 

 alluded to, a description is given with figures of octahedral 

 gold crystals, similar in form to those here mentioned. These 

 crystals were from the Princeton mine in Mariposa county, and 

 they agree so closely in form and habit with those here figured 

 as to leave no doubt that the planes, if determined by meas- 

 urement, would be found to have the same symbols. 



Art. XVI. — Classification of the Cambrian System of North 

 America ;* by Charles D. Walcott. 



The formations included within the Cambrian system, in 

 this paper, are those characterized by the predominence of the 

 types of the "First Fauna "f of Barrande and such additional 



* Read before the National Academy of Science at Washington, D. 0., April 

 23d, 1886. 



f The " First " or " Primordial Fauna" of Barrande, as found in North America, 

 is characterized by a trilobitic fauna which, in the presence of the genera Agnos- 

 tus, Paradoxides, Olenellus, Dicellocephalus, Ptychoparia and their allied genera, 

 distinguishes it from the succeeding Lower Silurian (Ordovician) fauna. 



