266 CENTRAL MINERAL REGION OF TEXAS. 



than is afforded by differences in the immediate environment, affecting in 

 unlike manner a general cause. 



These are the only safe deductions from our present knowledge of the 

 tripartite Burnetan System. Mr. Lawson, in his territory, inclines to the 

 theory of a Laurentian System equivalent, in effect, to our Lone Grove Series, 

 and he thinks that he has made out a great unconformity between this and 

 his succeeding " Keewatin Series," by which name he designates a compre- 

 hensive lithologic aggregate corresponding in composition not only with our 

 Long Mountain and Bodeville series, but including as well the members of our 

 succeeding Fernandan System. 



Irving, on the other hand, has correlated the rock groups and unconformi- 

 ties of the Archaean strata of nine different areas in the Lake Superior re- 

 gion, and his tabulation agrees essentially with the view here taken, that, 

 speaking only of the Archaean of Central Texas, the first pronounced un- 

 conformity yet fully recognized occurs at the summit of the Bodeville Series, 

 which is, lithologically, the equivalent of the second (in ascending order) of 

 the five divisions of Lawson's Keewatin Series.* 



With very few unimportant exceptions, each trend of the known irruptives 

 represents the course of one .of several uplifts of well ascertained geologic date. 

 Knowledge of this fact makes it, in many cases, comparatively easy .to deter- 

 mine upon the ground which individual injections of granite have been con- 

 temporaneous with the folding of the Fernandan strata. Or perhaps it would 

 be wiser to say that it is not always difficult to determine which irruptions 

 have surely been of earlier or later age. This is especially true of the sub- 

 sequent outbursts, for, if traced far enough, they will be found, as intrusives 

 or otherwise, environed by younger strata. So much it is easy to postulate, 

 but it is a very difficult thing to put down exact lithologic criteria by which 

 one may distinguish at a glance the granites of the different uplifts. The 

 writer finds that he is able to pick out from the general collection, with some 

 degree of accuracy, certain representatives of each of the different geologic 

 periods, of which at least six are indicated by the record. As yet, however, 

 no principles of classification have been established upon which the relative 

 ages of these rocks can be determined by texture alone. The general im- 

 pression left after much experience in the field and office is that the intimate 

 study of thin slices will develop a basis for such a grouping. Each success- 

 ive irruption, barring the youngest, has been invaded by later outbursts 

 which have modified locally the condition of the original mass. Examples 

 of this kind are abundant near Lockhart Mountain. A very common effect, 



Classification of Early Cambrian and Pre-Cambrian Formations. By R. D. Irving. 

 Seventh Annual Report United States Geological Survey (Powell), 1885-6. Washington, 

 1888, pp. 440, 441. 



