Si Scientific Intelligence. 



10. Boletln del Institute- Geologico cle Mexico, Num. 13, Las 

 JRhyolitas de Mexico. Primera parte, por Ezequiel Ordenez, 

 Mexico, 1900, 4°, 75 pp., 1 map. — This bulletin is the first of a 

 series to be issued on the eruptive rocks of Mexico. The aim of 

 the series is to make known the great number of varieties of 

 rocks, their geographical distribution and field relations, their 

 physiographic features, and economic importance as building 

 material and soil. It is also expected to bring about uniformity 

 in the petrographic nomenclature of Mexico. The first part of 

 the bulletin is devoted to historical notes, principal subdivisions 

 of the rbyolite family, macroscopic characters of the rhyolites and 

 geographic distribution of the rhyolites in Mexico. The second 

 and greater part is devoted to the description of various localities 

 under the following four heads : (1) The " Bufas," (2) The Nava- 

 jas, (3) Other regions of the Central Plateau, (4) The western 

 Sierra Madre. The term " bufa," for many years applied by the 

 miners in the region about Guanajuato and Zacatecas to curiously 

 shaped masses of rock crowning the summits of prominent points, 

 is adopted by the writer to designate these isolated erosion rem- 

 nants of rock of a rhyolitic nature. The bulletin closes with a 

 chapter upon the age of the rhyolites and a resume in French. 



Points of general interest are: the great distribution of the 

 rhyolites, surpassed only by the andesites, in a belt reaching 

 southward from the northern boundary of the country through 

 the western Sierra Madre into the Central Plateau to the vicinity 

 of the city of Mexico ; the greater part of them were erupted 

 from the end of" the Miocene to the middle of the Pliocene. The 

 succession of Tertiary eruptives, based on the evidence thus 

 far presented, is: (1) Granites-Gran ulites, (2) Diorites-Diabases, 

 (3) Andesites-Dacites, (4) Rhyolites, (5) Dacites- Andesites, (6) 

 Basalts-Basaltic andesites. There are two modes of occurrence, 

 namely, in the western Sierra Madre from fissures parallel to 

 lines of relief, and in the Central Plateau from local vents. The 

 intimate connection between the geologic structure and the erup- 

 tive rocks of the western United States and the mountains of 

 Mexico is also of interest. h. h. ». 



11. Brief notices of some recently described minerals. — Conch- 

 ite. This name has been given by Agnes Kelly to the calcium 

 carbonate forming a considerable part (often associated with cal- 

 cite) of the calcareous secretions of molluscan shells. This form, 

 unlike aragonite to which it has been referred (G. Rose), is uni- 

 axial and optically negative, as is calcite, and yet it differs from 

 the latter mineral in some other characters. The most conspicu- 

 ous difference is in the specific gravity, which is 2*87 f or conchite, 

 and 2*71 for calcite. It shows no twinning and has higher refrac- 

 tive indices; thus e is 1*524 for conchite and 1*486 for calcite. 

 Conchite is an unstable compound changing into calcite at 300° 

 to 310°. Various natural incrustations (Karlsbad, Yellowstone, 

 etc.) were found to consist of conchite; here also belongs the 

 Flos Ferri.— Min. Mag., xii, 363. 



