Dr. G. J. Hinde — Organic Origin of Chert. 443 



In all the sections of Chert which I had prepared sponge- 

 spicules are present ; in some their traces are indistinct and objection 

 might be raised that the few forms visible are insufficient to justify 

 the conclusion that the rock has been derived from these organisms. 

 In other slides of precisely the same kind of rock in outer appear- 

 ance, the spicules are so abundant that it must be conceded that, as 

 Prof. Sollas remarked of some of Prof. Hull's specimens, they make 

 up the larger part of the Chert. But even in the best-preserved 

 specimens of the Irish Chert, the spicules have suffered from the 

 influences of fossilization, and many are passing into various stages 

 of dissolution, and there are good grounds for supposing that in the 

 specimens which only show the spicules faintly, they have become 

 nearly obliterated through subsequent changes in the rock. 



I have discovered evidence strongly confirmatory of this view, in 

 the nature of the porous crust, which, as ah'eady mentioned, results 

 from weathering action on the surfaces of beds of Chert. In most 

 cases, this crust appears to consist only of minute siliceous grains, 

 but under favourable conditions of preservation, it is seen to be 

 composed of innumerable minute rod-like sponge-spicules, inter- 

 crossing, and as it were felted together irregularly in the plane of 

 the bedding of the rock. Also the layers of porous siliceous rock, 

 which, as already noticed, in some cases appears to take the place 

 of the compact Chert in the limestone, are likewise seen to consist 

 of matted masses of minute spicules. 



In the cases just mentioned, the evidence in favour of the view 

 that the silica of the Chert is derived from the accumulation of 

 the skeletal elements of siliceous sponges, and not as a direct deposit 

 of this mineral from solution in sea-water, is demonstrable and 

 conclusive. We can see in these instances that the silica of the 

 Chert is distinctly of organic origin ; the sponges have by vital 

 action secreted the silica to form their spicules, and whilst some of 

 these still retain their forms, others have been partially or com- 

 pletely destroyed to form the siliceous matrix. These spicular 

 crusts also prove the presence of these bodies in the central mass 

 of the beds of Chert, even when they cannot be distinguished in 

 microscopic sections. 



The preservation of the sponge-remains in the Irish Carboniferous 

 Chert is much less favourable than in the corresponding Chert-beds 

 of the Yoredale series in Yorkshire and North Wales ; the spicules 

 are far more eroded, and consequently it is more difficult to determine 

 their characters. They appear to be nearly entirely minute acerate 

 and cylindrical spicules varying from '3 to '9 mm. in length, and 

 from -017 to -12 mm. in thickness. Most of these spicules probably 

 belong to the Monactinellidee, though some of the larger forms mfiy 

 be those of Tetractinellid sponges. Occasionally slender filiform 

 spicules are present which attain a length of 2 mm., though only 

 •05 mm. in thickness. 



Not a single entire_ sponge has, as yet, been discovered in any of 

 the Chert-beds referred to, and though masses of rock reaching, as 

 w^e have seen, an estimated thickness of 100 to 150 feet (30-45 m.) 



