/Jt6 



-i#- Dr. Hinde on the Microscopic Structure 



Some of the rods are of a green colour, and nearly opaque to 

 transmitted light ; and many of these greenish rods occupy the 

 axial spaces of others, and are enclosed by a coating of a lighter 

 mineral. In the interspaces between the rods, and sometimes 

 also apparently in-filling cavities where the rods have been, there 

 are numerous nearly transparent minute discs or small spherular 

 bodies. Many of the transparent rods and most of the small 

 spherular bodies are negative to polarised light, but in others the 

 tints of chalcedony are indicated between crossed Nicols. A 

 section of the bluish nodules, or so-called flints, exhibits sub- 

 stantially a similar structure to the harder portions of the white 

 rock, but there seems to be in the nodules a greater quantity of 

 chalcedonic silica and also of the greenish rods. 



The main interest of this Malm rock is in the nature of these 

 rods, and tliere is no doubt whatever that they are the spicules of 

 siliceous sponges ; and consequently that the rock itself is mainly 

 of organic origin, and due to the living action of sponges m sepa- 

 rating the silica in solution in the sea-water to form their skeletons. 



The proof of these rods being the spicules of sponges rests on 

 the similarity of their structure and form to the spicules of 

 existing sponges. Of course in considering their present chemical 

 natme we must take into account the changes which the original 

 silica may have undergone during the process of fossilization. 

 In the spicular skeletons of existing sponges the siliceous material 

 is, as all are aware who have seen these objects under the micro- 

 scope, of a brilliant transparent aspect ; it is also negative to 

 polarised light, thus indicating that the silica is in a colloid con- 

 dition, as in opal, and readily soluble in heated caustic potash. 

 In this condition, however, it is unstable, and, under the influ- 

 ences to Avhich it is exposed in the rock, liable to change to the 

 more stable chalcedony or quartz. As a rule, most of the silica 

 in fossil sponges is now either chalcedonic or crystalline ; but 

 the spicules in this Malm are still for the most part in the con- 

 dition of opal, though they have lost that brilliant appearance so 

 characteristic of recent sponge-spicules, and, instead of being 

 homogeneous throughout, the silica appears to be separating into 

 minute granules and spherules. 



It is somewhat difficult to ascertain the complete forms of these 

 minute spicules when, as in the Malm rock, they are irregularly 

 mingled together in a hard matrix, and can only be stiidied in 

 thin sections in which fragments merely are shown. It fortu- 

 nately happens, however, that under certain conditions the rock 

 in places loses its cementing materials, and becomes powdery and 

 incoherent, so that by a little careful manipulation the spicules 

 can be isolated, retaining their forms fairly complete. From 

 some of this decayed rock or rotten stone I obtained a variety of 

 diflerent forms of spicules. 



