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3. Instructions on the Observations which • should be 

 made in case Volcanoes or Evidences of Volcanic 

 Action should be met with. By J. W. Judd, F.G.S. 



Should any volcanic rocks be met with, the following sugges- 

 tions may aid the observer in directing his attention to the most 

 important points in connexion with them. 



I. If the rocks have a fresh appearance, and are of com- 

 paratively recent origin, the following circumstances concerning 

 them should be particularly noted : — 



A. Lava Streams. Concerning these should be recorded, — 



a. Dimensions. Distance from point of origin ; breadth at 

 various parts of course ; thickness, so far as it can be determined, 

 and especially as affected by the accidents which the current 

 meets with in its flow. 



b. Slope over which they flow. This should be measured at 

 various points with a chnometer if possible, and, at points where 

 the inclination suddenly changes, any variations in the dimensions 

 or other characters of the current should be carefully noted. 



c. Surfaces of lavas. Attention should be paid to the features 

 presented by these, whether smooth and *' ropy," or bristling and 

 scoriaceous. 



d. Texture. Note especially if the rock of the current be 

 porphyritic, compact, globular, concretionary, pumiceous, giassy, 

 sphserulitic, or coarsely crystalline. If the rock presents ribboned 

 or banded structures, observe, if possible, the relations of these to 

 the direction of flow of the stream. When the rock exhibits 

 transitions from one texture to another, collect series of specimens, 

 illustrating the gi'adation. Note especially changes between the 

 surface and interior of current, or those taking place at different 

 points of its course. 



e. Structure. All peculiarities of jointed, and especially of 

 columnar, structures are worthy of being recorded. Note the 

 features presented by the upper and lower part of the current, 

 and any changes in its course ; also if columns be divided by 

 transverse joints, and the features presented by these, &c. 



f Chemical and mineralogical constitution. If the appearance 

 of the rock does not suggest at once the class to which it belongs, 

 and the component minerals cannot be detected with a lens, 

 recourse may sometimes be had to a determination (even roughly) 

 of its specific gravity. 



g. Sometimes lavas contain large masses of included minerals. 

 These are very interesting, and should be carefully collected. 



h. Cavities, or air bubbles, in -comparatively recent lavas, are 

 frequently found coated with beautifully crystaUised minerals. 

 And when the rocks are of older date, the similar cavities may 

 be lined or filled with crystals of zeolites and other minerals. 



