Br. Walcotfs Cambrian Geology & Palceontology. 25 



FlG. 2. — Matrix of conglomerate, Cilgwyn. Containing subangular quartz 



crystals (c) and grains of felsite (a) in an abundant sericitic matrix (b). 



Natural light, x 18. 

 ,, 3. — Quartz grit, Cil-y-Coed. Containing angular quartz crystals (c) 



embedded in a felspathic groundmass ; small grains of felsite (rather 



decomposed) (a) ; occasional felspars (6) ; and abundant iron-ore (d). 



Natural light. x 18. 

 ,, 4. — Quartz grit, Bwlch-y-llyn. Containing angular quartz crystals (a} 



embedded in an abundant matrix (c) now largely decomposed into sericite. 



Abundant iron-ore (b). Crossed nicols. x 18. 

 ,, 5. — Sheared quartzite, Cil-y-Coed. Crossed nicols. x 18. 

 ,, 6. — Sheared quartzite, Bwlch-y-llyn. Crossed nicols. x 18. 

 (To be concluded m our next Number.) 



V. — De. Chaeles D. Walcott's Cambeian Geology and 



Paleontology. 1 



By V. C. Illing, F.G.S. 



TO the student of Cambrian geology, the writings of C. ~D. "Walcott 

 are always matters of enlightening study, not only for the 

 matter they contain but also because they combine that admixture 

 of stratigraphy and palaeontology in which the latter, though 

 accorded a prominent position, is always used to the full to subserve 

 the wider claims of the former, i 



In a consideration of the series of papers on Cambrian Geology 

 and Palaeontology in the Smithsonian Miscellaneous Collections, the 

 subjects covered are so varied and extensive that only a few of the 

 more salient points can be considered. Perhaps it will be most 

 convenient to discuss the material under the two general considera- 

 tions of the more purely palaeontological and the stratigraphical. 



Among a series of new forms, some of the most remarkable are 

 a number of Merostomata, Malacostraca, Holothuroidea, and Annelids, 

 found in the Burgess shale of the Stephen Formation in British 

 Columbia. In many cases the impressions of the organic structures 

 are beautifully preserved in the fine-grained material, a fact to which 

 full justice is done in the figures. The Merostomata are particularly 

 interesting in this connexion as indicating to what a degree of 

 development these Middle Cambrian faunas had attained. But to the 

 stratigrapher it is the trilobites to which the main interest generally 

 attaches, and among the various groups the Mesonacidse or Olenellidse 

 stand out in their interest and importance. This family, characterized 

 by its large head, large crescentic eyes, rudimentary facial sutures, 

 genal spines, and long and variable thorax, has now been divided 

 into a number of genera based mainly on variations in the thorax. 

 Walcott recognizes six main stages in the development in an order of 

 decreasing number of thoracic segments. 



1 . Nevadia Stage. The seventeen anterior thoracic segments are 

 of the usual type, but are followed by eleven primitive posterior 

 segments with spinous extensions. 



2. Mesonacis Stage. The first fifteen segments are normal, except 

 the third, which is enlarged, and the fifteenth, which has a median 

 spine. The ten posterior segments are normal in shape but small. 



1 Smithsonian Miscellaneous Collections, 1910-15. 



