Professor Epwarp Hutn—On the Nature and Origin of Beds of Chert. 79 
rotating the analyzer being to change the light brown or gray colour of the specimen 
under parallel Nicol’s prisms to dull or deep indigo when the prisms vere crossed. 
The effect was very similar to that presented by chalk flint under similar 
circumstances. 
The polariscope sometimes proved of assistance in bringing into view the 
organic forms, which, otherwise, were invisible or obscure. By its aid also small 
crystals, veins, and nests of calcite came into view. In rare instances calcite or 
crystalline quartz was found to have replaced the organic structure while the 
inclosing paste was in all cases gelatinous silica. 
Appearance of the Organic Forms under the Mucroscope.—In general the 
visibility of the organic structures issolely due to their being more transparent 
than the inclosing siliceous paste. This latter appears as a clouded or mottled 
brown translucent substance in which the organic forms are set, and they being 
either clear or colourless can be distinguished from the inclosing paste. This 
suggests the idea that during the process of transmutation the conversion of the 
carbonate of lime did not take place simultaneously throughout. As the lime of fossil 
shells is generally in a crystalline condition, and the inclosing paste amorphous 
this latter would probably be more rapidly transmuted into silica than the former. 
Sometimes the forms are clearly and sharply defined, at other times they are 
shaded off, but in general there does not appear to be any definite wall between the 
paste and its inclosed organic form, the original shell or skeleton having altogether 
disappeared.* 
Modes of Occurrence of Chert.—The modes of occurrence of chert, amongst the 
limestone rocks, are exceedingly varied and often indicate the pseudomorphic 
replacement of the original rock. When found in small masses it generally occurs 
in short lenticular beds with uneven surfaces abruptly terminating and lying in 
the planes of bedding. At other times the circumstances are entirely different, and 
the chert occurs in regular nodules of rounded fanciful forms, like flints in chalk, 
and planted transversely to the bedding of the rock; on the other hand I have 
never seen it assume the spongiform shapes of some chalk flints. 
When it occurs in large masses the chert is banded or bedded in the direction 
of the planes of stratification, and is generally found in alternate beds with lime- 
stone, but sometimes almost entirely free from this rock. At all times the beds 
are more or less abruptly terminated or lenticular, although occupying the same 
general stratigraphical position, When the rock weathers, the chert bands or 
nodules stand out from the surface often in fantastic shapes, being less liable to 
decompose than the limestone which accompanies it. Some truthful sketches of 
these forms will be found in the Explanatory Memoirs of the Geological Survey of 
Ireland. 
*Some of the slices which show foraminiferal structure resemble sections of the compact carbon- 
iferous limestone which, when sliced, is found to be full of foraminiferal shells; in each case the 
structure is preseryed, but in the former the carbonate of lime is replaced by silica. 
