PLATE XXYII. 



Fig. a. 



(Sp. No. 32903. Natural size.) 



Reproduction of a very fine grainetl, possibly vitreous, amydaloidal basalt. The auiygdaloidal 

 cavities show very little contortion. Tlie amygdules consist of white quartz, pink feldspar, aud 

 dark-green chlorite. Compare this iiguro with photomicrograph fig. B, PI. XXV. (Described, p. 102.) 



Fig. B. 



(Sp. No. 32910. Natural size.) 



Colored reproduction of the pseudoamygdaloidal phase of the siderite-quartz matrix which 

 occurs iu places between the ellipsoids. The original matrix was fir^t replaced in the zone of weath- 

 eriun- by siderite. Deep burial of the rock resulted iu the mashing of the sideriteand thesubseijueut 

 replacement of a great portion of it by silica, leaving a few oval areas uusiliciiied. Brought into the 

 zone of weatheriug again by erosion, these siderite areas are removed on the weathered surface 

 giving a scoriaceous appearance to it, as may be seen on the figure. (Described, p. 135.) 



Fig. C. 



(Sp. No. 33507. Natural size.) 



Fiue-grained volcanic clastic. The water-deposited character of this specimen is unquestion- 

 able. It can be traced in the field down into a coarse bowlder conglomerate. The fragmeutal nature 

 can only be seen under the microscope in the coarser-grained portions. The finer-grained material 

 represents apparently the excessively fine-grained mud derived from the trituration of tlie coarser 

 fra<''meDts. The eolian-deposited sands and dust have essentially the s.amo appcurauce as the specimen 

 here represented. The microscope, however, shows the very angular character of the fragments iu 

 the coarser-grained portions. The very fine eolian-deposited dust can not be distinguished from that 

 which has been deposited through water. It is sometimes very difficult to determine the nature of 

 rocks of this fiue-grained character. (Described, p. 144.) 



