Williams — Occurrence of Paleotrochis in Mexico. 335 



Art. XXXYI. — On the Occurrence of Paleotrochis in Vol- 

 canic Rocks in Mexico ; by Henry S. Williams. 



Introductory Note. — This article was written, Nov. 22, 

 1897, at the time of the study of the specimens sent by Dr. 

 Duges, but for various reasons was not then published. A 

 discussion in the Geological Society last December led the 

 writer to refer to these specimens as proof of the non-organic 

 nature of Paleotrochis. Later the Mexican specimens, together 

 with the manuscript copy of this paper, were sent to Mr. 

 Walcott and then to Mr. Diller in confirmation of the inor- 

 ganic origin, and association with igneous rocks, of Paleotrochis. 

 It is thought best to publish these facts for what they are 

 worth, in connection with Mr. Diller's thorough study of the 

 subject as presented in the following paper. 



Some specimens sent to Yale College for identification, by 

 Dr. Alfred Duges of Guanajuato, Mexico, throw some light 

 upon the nature of the peculiar forms described by Emmons 

 under the name of Paleotrochis. The specimens come from 

 what appears to be an old eroded volcanic cone made up of 

 "Porphyre altere," "Peckstine rouge" and volcanic tuffs, 

 situated northeast of Guanajuato in the Santa Posa Mountain 

 range. Among the specimens are several fragments of much 

 altered f ragmen tal as well as massive volcanic rocks. One 

 specimen is filled with amygdules. 



The majority of the specimens are composed of milky quartz 

 of the shape called Paleotrochis by Emmons, with variations 

 in form which are particularly suggestive. These suggestive 

 variations are the imperfect form, one-sided, or as if made up 

 of several of the conical halves of typical paleotrochis joined 

 together at their bases, some of the cones being very broad 

 and nearly flat, but each having the appearance of accretion 

 from the small end which in the lower cones is near the center. 

 When thus nearly flat the lines of concentric accretion are 

 evident, while the cones with less angle of slope show the 

 radiating lines more conspicuously. The suggestion furnished 

 by their study is that they were formed in a manner similar to 

 that of the formation of ice columns under a loose soil, which 

 at about freezing temperature sometimes grow to several inches 

 in length. The lengthening of the column of ice, in the latter 

 case, appears to be due to additions of ice forming as soon as 

 the cool atmosphere is reached, while the temperature of the 

 soil is still enough below freezing to permit the water to be 

 drawn up by capillary flow to the surface as the freezing 

 removes it from the surface immediately below. 



