1 66 THE JOURNAL OF GEOLOGY. 



because of the greater or less completeness of the material 

 collected, than by reason of their actual scarcity or abundance in 

 nature. 



The chief feature of the report consists in the systematic 

 description of the lavas of Colombia based upon their microscop- 

 ical investigation in conjunction with their chemical analysis. The 

 second part of the report is devoted to a description of the rocks 

 in connection with their geographical distribution. It is to be 

 regretted that the geological relations of the rocks with one 

 another are not furnished at the same time. 



The rocks are first discussed from a mineralogical standpomt, 

 their mineral composition and structure being taken as the basis 

 of classification within the general group of extrusive igneous 

 rocks, to which they all belong. They are all embraced within 

 the families of andesite and dacite, as defined by Rosenbusch. 

 They present a chemical series grading from rocks relatively 

 poor in silica and rich in lime and magnesia with sodium consider- 

 ably in excess of potassium, to those comparatively rich in silica, 

 and poor in lime and magnesia, but with sodium still in excess of 

 potassium. The lower limits approach basalt, and the upper 

 limits border rhyolite. 



The same gradual transition exists in the mineralogical com- 

 position. At one end are pyroxene - andesites with accessory 

 olivine, the feldspars being rather basic plagioclase. These 

 pass into pyroxene - andesites without olivine, and into horn- 

 blende - pyroxene - andesites, and hornblende - andesites, and with 

 increasing amounts of quartz into dacite, or quartz - andesites. 

 In the dacites the feldspars are : plagioclase, approaching albite, 

 and sanidine; while biotite becomes prominent among the ferro- 

 magnesian minerals. 



In considering the classification of such a series of rocks, 

 since their mode of occurrence is the same throughout, namely, 

 that of lava streams, Kiich finds the grounds of classification to 

 be : chemical composition, mineral composition and structure. 

 Of these, chemical composition is undoubtedly that which under 

 like conditions of solidification controls the mineral and structural 



