On Inoceramus Limestone. Bij F. Chapman. 3 



ments being the limiting thicknesses of the shells noticed. Sub- 

 jected to the influence of weathering, the rock shows a tendency to 

 exfoliate in large pieces, the surfaces of which are not always in 

 perfect parallelism with the sedimentary planes, but often highly 

 curved. 



Microscopic Details. — -In thin sections under the Microscope the 

 Inoceramus prisms are seen to occupy a large part of the field. 

 They show a rudely parallel arrangement, and are cemented together 

 by a ferruginous and calcareous matrix. As before stated, they 

 measure from about 3-7 mm. in length. The carbonate of lime in 

 the matrix approximates to about 10 p.c. of the whole, so that it 

 reduces the proportion of shell-material to about 50.p.c. of the rock- 

 mass. Interspersed amongst the shell -prisms are numerous per- 

 fectly angular fragments of clear quartz, and an occasional piece of 

 felspar (pagioclase). The quartz grains sometimes contain needles 

 of rutile. The Inoceramus prisms vary from clear calcite to pale 

 brown granulated calcite. In nearly all may be seen the accre- 

 tionary bands of growth of the original shell-structure, at right 

 angles to the length of the prismatic axis. They are horn-brown 

 in colour and of varying translucence. Numerous differential 

 cracks break up the prisms at right angles to their length into 

 quadrate sections, but the original rhombohedral cleavage is always 

 more or less distinct. Between crossed nicols most of the prisms 

 behave as single crystals of calcite, and show a straight extinction. 

 The boundaries of the prisms are in all cases sharply outlined 

 in dark brown, owing to a surface deposit of ferruginous material. 

 Sometimes there is a thin superficial outgrowth of calcareous 

 crystals upon the prisms, and, more rarely, corrosion of the surface 

 of the prism has taken place. Here and there the prisms show a 

 finely granulate appearance, their internal structure having been 

 physically reconstructed. In some cases there is evidence of slight 

 dolomitisation. 



Conditions of Deposition. — As a general conclusion the Queens- 

 land Lower Cretaceous rocks may be said to have been deposited 

 in fairly shallow water, as seen particularly by the current-bedded 

 structure ; the Inoceramus shells in all probability having formed 

 part of the beach-material of that ancient shore-line. 



The Lower Cretaceous rocks in the northern part of Queensland 

 rest upon gneisses and granites ; the Jurassic series (Ipswich forma- 

 tion) being wanting. At a distance of about 150 miles to the north 

 of the locality which furnished the limestone in question, there is 

 a massif of granite with a limited area of gneiss at the foot. Against 

 these granitic rocks the shore deposits of the Lower Cretaceous sea 

 were undoubtedly laid down. The angular quartz grains and occa- 

 sional felspars previously referred to, as forming a large proportion 

 of the Inoceramus limestone, would therefore most likely be derived 

 directly from the detritus of the highlands in the vicinity, and 



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