[GNEOUS FORMATIONS. 311 



Tliis composition corresponds closely to what might be expected from 

 the mineralogical constitution. It seems as if the orthoclase must be rich 

 in soda, as there is little determinable plagioclase present It is worthy of 

 note that by exchanging 6 per cent of silica for iron oxide this analysis 



becomes very oearly Like tl nalyses of tin- Table Mountain basalt. No 



positive evidence is known, however, indicating any close relationship in 

 origin between the two rocks. 



ROCKS OF THE DENVER FORMATION. 



General statement. TliC SCll'l 1 IK'H tS of tllis I'l ll'l I la t i< 111 a IV tO 1>C das>ed .■ I S 



clays, tuffs, sandstones, grits, and conglomerates, with intermediate mem- 

 bers illustrating transitions in every direction. In Chapter III, Section II, 

 will be found descriptions of the field relations of these types and data as 

 to their distribution and local occurrence. 



The present description has a twofold object. On the one hand, the 

 rock names and characterizations found in the geological descriptions are 

 to be explained and justified, in so far as they are based upon the results 

 of a petrographical examination. This applies to rocks called tuffs, or 

 sandstones, that are said to be made up almost entirely of the debris of 

 volcanic rocks, a tact by no means obvious in many cases. On the other 

 hand, the conglomerates contain certain andesitic pebbles, which anyone at 

 all acquainted with the outer appearance of volcanic rocks would at once 

 recognize as such. A concise petrographical description of these pebbles 

 is given, to prove the great and interesting diversity claimed. The sources 

 of these rocks are not known: hence detailed descriptions are not calledfor. 



VOLCANIC TUFF. 



General character. Tll6 teiTJQ tllff is llLTC applied tO H Stratified l'ock Com- 

 posed of angular mineral or rock particles nearly all of which are of 

 some volcanic source. This does not necessarily imply that these particles 

 fell into the sea as ashes ejected from some neighboring vent. They may 

 have been washed into the ocean from land regions where they fell as 

 ashes, or they may have come from rapid disintegration of a lava. It is 

 simply intended to characterize certain sediments in distinction from others 

 containing a predominant amount of worn and rounded grains derived 



