F. N. Guild — Eruptive Rocks in Mexico. 175 



ous and basaltic-like lava from Popocatepetl. These are 

 almost identical in chemical composition yet have given rise to 

 dissimilar mineralogical developments. The one has developed 

 abundant hornblende, the other pyroxene. This of course 

 is easily explained on the ground that the consolidation 

 has taken place under diverse conditions, the one being erupted 

 in laro-e masses which formed mountain ranges and so cooling 

 very slowly and under great pressure, the other in small out- 

 flows which in some cases barely spilled over the rim of the 

 crater. Farrington* in comparing the rocks of Popocatepetl 

 and Ixtaccihuatl speaks of the remarkable fact that they differ 

 completely in character, the one being hypersthene andesite 

 and the other a quite dissimilar appearing hornblende ande- 

 site. It is quite possible that in this case as in the one men- 

 tioned above a similar chemical composition is masked by a 

 dissimilar mineralogical structure. This view is further sub- 

 stantiated by comparing the analysis given above (JSTo. 1) with 

 an incomplete analysis of hornblende andesite from Ixtaccih- 

 uatl made by Felix and Lenk.f As is well understood, horn- 

 blende requires for its formation unusual conditions of pressure, 

 etc., while the pyroxenes do not. The recent lavas of Popo- 

 catepetl are more acid in composition than the old. The 

 few basalts are mostly covered up by andesitic outflows. The 

 older andesite of Tlamacas is of a more basic type than those 

 collected about the rim of the crater. 



Volcanic sand and ashes are developed in enormous quanti- 

 ties around the base of the older mountains in many places in 

 Mexico, and constitute one of the chief sources of the great 

 fertility of the soil. The writer has been told that after the 

 coffee plantations in that country have become covered for 

 many square miles with a thin mantle of gray ash from the 

 volcanoes, contrary to the expectations of the haciendadoes the 

 soil has been improved and better yields experienced. The 

 older volcanic sands frequently become cemented and consti- 

 tute a material of sufficient strength to be used as a building 

 stone. It is often so soft that it can be worked into various 

 shaped bricks by means of a hatchet, and not infrequently 

 becomes harder on exposure to the air. The texture varies 

 from a fine-grained stratified deposit like that described from 

 Las Calderas, to rounded semi-pumiceous grains somewhat 

 larger than peas cemented by finer material. Many of the 

 varieties frequently have a peculiar, not unpleasing and rusty 

 appearance. Intermediate varieties sometimes show oolitic 

 structures. 



*Op. cit., p. 109. 



f Si0 2 61-24, A1 2 3 18-32, Fe 2 3 and FeO 6-17, MgO 376, CaO 5-06, Na 2 

 3-15, K 2 2-37, H 2 0"67, Felix und Lent, Btr. Geol. Mex., II, p. 229, 1899. 



Am. Jour. Sci.— Fourth Series, Vol. XXII, No. 128.— August, 1906. 

 12 



