552 shaw: NEW AREA of carboniferous 



The occurrence elsewhere of so extensive a mass of andalusite 

 rock as that in the pre-Cambrian of the Inyo Range appears 

 to be unknown. It is provisionally suggested that the andalu- 

 site mass originated from the volcanic porphyry through pneu- 

 matolytic metamorphism sequent upon the granitic intrusions 

 that occur in the range. Since examining this andalusite rock 

 the writer has had occasion to study in detail closely similar 

 andalusite rock in the Rochester district, Humboldt County, 

 Nevada, where a pneumatolytic origin is unmistakable. A 

 series of Triassic rhyolites is cut by a network of dumortierite 

 veinlets carrying sporadic tourmaline, and the intervening rock 

 has been altered to an aggregate of quartz and poikilitic 

 andalusite, in places carrying considerable dumortierite 

 (Al 8 HBSi 3 2 o), a mineral closely related to andalusite. This 

 metamorphism closely followed the intrusion of post-Jurassic, 

 probably Cretaceous, granitic rocks. Andalusitic alteration 

 of Tertiary lavas, an alteration which evidently took place 

 under conditions of lower temperature (under "hydro thermal" 

 conditions) than those indicated for the California and Nevada 

 occurrences, has been described by Butler. 3 who pointed out 

 that metamorphism of this kind had not previously been 

 recorded. 



GEOLOGY. — A new area of Carboniferous rocks with some coal 

 in the north end of the Gulf embayment. 1 Eugene Wesley 

 Shaw, Geological Survey. 



The geologic maps of the southern end of Illinois and ad- 

 joining territory show the area of Cretaceous and later deposits 

 known as the Gulf or Mississippi embayment as extending north- 

 ward across one row of counties in Illinois and terminating at a 

 more or less regular line convex to the north, along what was 

 formerly a portion of the valley of the Ohio. On the north side 

 of this abandoned valley is the range of hills of Paleozoic rocks 

 that crosses southern Illinois from the Ozarks at Sainte Gene- 



3 Butler, B. S. Geology and ore deposits of the San Francisco and adjacent 

 districts, Utah. U. S. Geol. Survey Prof. Paper 80: 78-82. 1913. 

 1 Published by permission of the Director, U. S. Geological Survey. 



