ARCH.EAN GROUP. 255 



where to seek conclusive evidence. The discussion which follows is arranged, 

 as is most appropriate, in the order of geologic history, beginning with the 

 most ancient strata. 



I. ARCH^AN GROUP. 



Although it is one of the most important points, economic as well as geo- 

 logic, to determine whether any of the rocks of the region are of Archaean 

 date, and although full enough evidence has probably been collected to 

 answer the question, it is hardly possible now to present the matter fully, 

 because there remains too much work to be done in the office in the study of 

 rock slices, and in classifying the scattered observations and correlating them. 

 New questions, which must be worked out in the field, have also been opened. 



The writer has, however, become convinced that the basal system, and 

 probably two systems, of strata in the district of Central Texas may be 

 properly classed as Archaean members, provisionally placed as the homologues 

 of the Laurentian and Ontarian systems, respectively, of other areas.* 



This opinion must stand or fall, in accordance with the preponderance of 

 the evidence, when the intimate study of the collections has been completed ; 

 but the expression of it here is based upon facts of structure which have 

 been most carefully worked out in the field. From a stratigraphic stand- 

 point, the position taken seems well fortified by facts which do not admit of 

 any other interpretation. Such investigations as have been possible to date 

 lead me to believe that microscopic study of the rocks will only confirm this 

 judgment. 



This is the first reference of these rocks to the Archaean by any one who 

 has assumed individual responsibility for the statement. Mr. Walcott, how- 

 ever, is the only scientific authority who has positively denied the existence 

 of Archaean rocks in our field. As the time he devoted to the study of the 

 region was limited, he probably saw but little of the outcrops here included, 

 although his published section of Packsaddle Mountain was made very near 

 some of the exposures to be described herein. 



1. BURNETAN (LAURENTIAN?) SYSTEM. 

 A glance at a good topographic map of Central Texas will show a decided 



*It is unfortunate that Dr. J. D. Dana and Mr. Andrew C. Lawson have recently proposed 

 the name Ontarian for two very different systems. Courtesy and the more general rules of 

 nomenclature give the preference to the former, as the first publisher ; but Mr. Lawson's 

 application is to a recognized and increasing necessity in terminology, demanding a Canadian 

 or geographically restricted term. Dr. Dana applies it to the Upper Silurian, for which, 

 perhaps, a more fitting term may be found, and he does not seem to urge strenuously the 

 adoption of Ontarian, if any equally suitable name be suggested. As used herein, therefore, 

 the term Ontarian is a systemic designation for Post-Laurentian Archaean strata. 



