Side-Light upon. Coal Fonnation. — Gresley. 79 
perplexing- question — what are these interstratified sheets of 
clayey material in the coal, and how did they get there? 
9. Clearly, then, the coal is still full of objects of the greatest 
interest to the patient observer and investigator, and if the 
foregoing facts and suggestions have no more effect upon the 
question of coal-formation, than a roninder that there are dif- 
ferent ways of looking at and into the subject, this communica- 
tion will not have been prepared in vain. 
Mischief has been done in the past, and but little benefit 
10 science can come in the future, from making isolated facts 
basis for generalizations. Remember too, that it is so long- 
ago since the coal-period, that the greatest caution is necessarv 
in attempting to draw conclusions from facts. Leave deduc- 
tions for future generations. Avoid "must he's." Experi- 
ment, observe, record phenomena. 
Explanation of Plate II. 
Fig. I. A variety of "Rods" in situ in a bit of "Pittsburg" coal: 
the upper rod contains spindle-shaped beads of milk-white substance; 
and a clear, pale, yellowish material (?ambrite) fills the cavities of the 
lowest rod on left. Magnified about 25 diameters. 
Fig. 2. The aspect of the cells surrounding some of these rods. 
X 150. 
Fig. 3. Transverse section of a rod surrounded by cells. From 
Penna. Anthracite. X 60. 
Fig. 4. Longitudinal section (somewhat oblique) of "tubes." com- 
posed of thick-walled fibres or bundles having gray cellular centres, 
and surrounded by gray cellular tifsue. From Lehigh anthracite, 
Penna. X i5- 
Fig. 5. Oblique section of tubes and matri.x of fig. 4. From Lehigh 
Anthracite, Penna. X 15. 
Fig. 6. Transverse section of tubes and intertubu'ar material. 
with portion of the epidermis (?) of the plant. 
Fig. 7. Transverse section of one of the same tubes. X .SO. 
Figs. 8, 9 and 10. Structural markings developed by polishing one 
horizontal surface and two sides (at right angles to one another) of a 
fragment of a bright pitch-coal lamina from the "Pittsburg" coal bed. 
X 300. 
Fig. II. Structural markings upon the vertical face of a thin scale 
of selenite occupying a crack in a pitch-coal lamina in the "Pittsburg" 
ccal. (Hydrocarbonaceous material has imprinted itself upon the scale 
of lime in such a way as to make a "nature print" of the vegetable 
structures contained in the coal.) X -^00. 
Fig. 12. Portion of several rows of (?) vegetable cellular structures 
